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Web applications are popular due to the ubiquity of web browsers, and the convenience of using a web browser as a client, sometimes called a thin client. The ability to update and maintain web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity, as is the inherent support for cross-platform compatibility. Common web applications include webmail, online retail sales, online auctions, wikis and many other functions.
In earlier computing models, e.g. in client-server, the load for the application was shared between code on the server and code installed on each client locally. In other words, an application had its own client program which served as its user interface and had to be separately installed on each user's personal computer. An upgrade to the server-side code of the application would typically also require an upgrade to the client-side code installed on each user workstation, adding to the support cost and decreasing productivity.
In the early days of the Web each individual web page was delivered to the client as a static document, but the sequence of pages could provide an interactive experience, as user input is returned through web form elements embedded in the page markup.
In 1996, Macromedia introduced Flash, a vector animation player that could be added to browsers as a plug-in to embed animations on the web pages. It allowed the use of a scripting language to program interactions on the client side with no need to communicate with the server.
In 2005, the term Ajax was coined, and applications like Gmail started to make their client sides more and more interactive. A web page script is able to contact the server for storing/retrieving data without downloading an entire web page.
Applications are usually broken into logical chunks called "tiers", where every tier is assigned a role. Traditional applications consist only of 1 tier, which resides on the client machine, but web applications lend themselves to a n-tiered approach by nature. Though many variations are possible, the most common structure is the three-tiered application. In its most common form, the three tiers are called presentation, application and storage, in this order. A web browser is the first tier (presentation), an engine using some dynamic Web content technology (such as ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP, Perl, Python, Ruby on Rails or Struts2) is the middle tier (application logic), and a database is the third tier (storage). The web browser sends requests to the middle tier, which services them by making queries and updates against the database and generates a user interface.
For more complex applications, a 3-tier solution may fall short, and it may be beneficial to use an n-tiered approach, where the greatest benefit is breaking the business logic, which resides on the application tier, into a more fine-grained model. Another benefit may be adding an integration tier that separates the data tier from the rest of tiers by providing an easy-to-use interface to access the data. For example, the client data would be accessed by calling a "list_clients()" function instead of making an SQL query directly against the client table on the database. This allows the underlying database to be replaced without making any change to the other tiers.
There are some who view a web application as a two-tier architecture. This can be a "smart" client that performs all the work and queries a "dumb" server, or a "dumb" client that relies on a "smart" server. The client would handle the presentation tier, the server would have the database (storage tier), and the business logic (application tier) would be on one of them or on both. While this increases the scalability of the applications and separates the display and the database, it still doesn't allow for true specialization of layers, so most applications will outgrow this model.
Business use 
An emerging strategy for application software companies is to provide web access to software previously distributed as local applications. Depending on the type of application, it may require the development of an entirely different browser-based interface, or merely adapting an existing application to use different presentation technology. These programs allow the user to pay a monthly or yearly fee for use of a software application without having to install it on a local hard drive. A company which follows this strategy is known as an application service provider (ASP), and ASPs are currently receiving much attention in the software industry.
Security breaches on these kinds of applications are a major concern because it can involve both enterprise information and private customer data. Protecting these assets is an important part of any web application and there are some key operational areas that must be included in the development process. This includes processes for authentication, authorization, asset handling, input, and logging and auditing. Building security into the applications from the beginning can be more effective and less disruptive in the long run.
In cloud computing model web applications are software as a service (SaaS). There are business applications provided as SaaS for enterprises for fixed or usage dependent fee. Other web applications are offered free of charge, often generating income from advertisements shown in web application interface.
Writing web applications 
Writing of web applications is often simplified by open source software such as Django, Ruby on Rails or Symfony called web application frameworks. These frameworks facilitate rapid application development by allowing a development team to focus on the parts of their application which are unique to their goals without having to resolve common development issues such as user management. While many of these frameworks are open source, this is by no means a requirement.
The use of web application frameworks can often reduce the number of errors in a program, both by making the code simpler, and by allowing one team to concentrate on the framework while another focuses on a specified use case. In applications which are exposed to constant hacking attempts on the Internet, security-related problems can be caused by errors in the program. Frameworks can also promote the use of best practices such as GET after POST.
In addition, there is potential for the development of applications on Internet operating systems, although currently there are not many viable platforms that fit this model.
Examples of browser applications are simple office software (word processors, online spreadsheets, and presentation tools), but can also include more advanced applications such as project management, computer-aided design, video editing and point-of-sale.
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- Web applications do not require any complex "roll out" procedure to deploy in large organizations. A compatible web browser is all that is needed;
- Browser applications typically require little or no disk space on the client;
- They require no upgrade procedure since all new features are implemented on the server and automatically delivered to the users;
- Web applications integrate easily into other server-side web procedures, such as email and searching.
- They also provide cross-platform compatibility in most cases (i.e., Windows, Mac, Linux, etc.) because they operate within a web browser window.
- With the advent of HTML5, programmers can create richly interactive environments natively within browsers. Included in the list of new features are native audio, video and animations, as well as improved error handling.
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- In practice, web interfaces, compared to thick clients, typically force significant sacrifice to user experience and basic usability.
- Web applications absolutely require compatible web browsers. If a browser vendor decides not to implement a certain feature, or abandons a particular platform or operating system version, this may affect a huge number of users;
- Standards compliance is an issue with any non-typical office document creator, which causes problems when file sharing and collaboration becomes critical;
- Browser applications rely on application files accessed on remote servers through the Internet. Therefore, when connection is interrupted, the application is no longer usable. However, if it uses HTML5 API's such as Offline Web application caching, it can be downloaded and installed locally, for offline use. Google Gears, although no longer in active development, is a good example of a third party plugin for web browsers that provides additional functionality for creating web applications;
- Since many web applications are not open source, there is also a loss of flexibility, making users dependent on third-party servers, not allowing customizations on the software and preventing users from running applications offline (in most cases). However, if licensed, proprietary software can be customized and run on the preferred server of the rights owner;
- They depend entirely on the availability of the server delivering the application. If a company goes bankrupt and the server is shut down, the users have little recourse. Traditional installed software keeps functioning even after the demise of the company that produced it (though there will be no updates or customer service);
- Likewise, the company has much greater control over the software and functionality. They can roll out new features whenever they wish, even if the users would like to wait until the bugs have been worked out before upgrading. The option of simply skipping a weak software version is often not available. The company can foist unwanted features on the users or cut costs by reducing bandwidth. Of course, companies will try to keep the good will of their customers, but the users of web applications have fewer options in such cases unless a competitor steps in and offers a better product and easy migration;
- The company can theoretically track anything the users do. This can cause privacy problems.
- According to Jonathan Zittrain, the online applications like Facebook and Google Apps have made the Internet become far more proprietary than early versions of Microsoft Windows.
See also 
- Software as a service (SaaS)
- Web 2.0
- Web services
- Web widget
- Single-page application
- Ajax (programming)
- What is a Web Application?  by Daniel Nations, former About.com Guide
- Comprehensive solutions for managing your data- Web Applications http://www.meddb.be/webapplications.aspx by MedDb
- Alex Chaffee (2000-08-17). "What is a web application (or "webapp")?". Retrieved 2008-07-27.
- James Duncan Davidson, Danny Coward (1999-12-17). Java Servlet Specification ("Specification") Version: 2.2 Final Release. Sun Microsystems. pp. 43–46. Retrieved 2008-07-27.
- "Dynamic HTML and XML: The XMLHttpRequest Object". Apple Inc. Retrieved 2008-06-25.
- Jeremy Petersen. "Benefits of using the n-tiered approach for web applications".
- "Top Tips for Secure App Development". Dell.com. Retrieved 2012-06-22.
- Multiple (wiki). "Web application framework". Docforge. Retrieved 2010-03-06.
- Multiple (wiki). "Framework". Docforge. Retrieved 2010-03-06.
- Multiple. "Offline Web applications - HTML5". WHATWG. Retrieved 2010-08-09.
- Brian Braiker (2008-05-02). "A Killer Product: Will closed devices like Apple's iPhone murder the Web?". Retrieved 2009-06-16. "Through historical accident, we've ended up with a global network that pretty much allows anybody to communicate with anyone else at any time. Devices could be reprogrammed by them at any time, including code written by other people, so you don't have to be a nerd to get the benefits of reprogramming it. [But] this is an historical accident. Now, I see a movement away from that framework--even though it doesn't feel like a movement away. [For example,] an iPhone can only be changed by Steve Jobs or soon, with the software development kit, by programmers that he personally approves that go through his iPhone apps store. Or whimsical applications that run on the Facebook platform or the new Google apps. These are controllable by their vendors in ways that Bill Gates never dreamed of controlling Windows applications. [...] That's the ironic thing. Bill Gates is Mr. Proprietary. But for my purposes, even under the standard Windows operating system from 1990, 1991, you write the code, you can hand it to somebody else and they can run it. Bill Gates has nothing to say about it. So it's funny to think that by moving in Steve Jobs's direction it actually ends up far more proprietary."
|Wikimedia Commons has media related to: Internet applications|
- HTML5 Draft recommendation, changes to HTML and related APIs to ease authoring of web-based applications.
- The Other Road Ahead — An article arguing that the future lies on the server, not rich interfaces on the client
- Web Applications at the Open Directory Project
- Web Applications Working Group at the World Wide Web Consortium (W3C)