  A large amount of data on the WWW remains inaccessible to crawlers of Web
search engines because it can only be exposed on demand as users fill out and
submit forms. The Hidden web refers to the collection of Web data which can be
accessed by the crawler only through an interaction with the Web-based search
form and not simply by traversing hyperlinks. Research on Hidden Web has
emerged almost a decade ago with the main line being exploring ways to access
the content in online databases that are usually hidden behind search forms.
The efforts in the area mainly focus on designing hidden Web crawlers that
focus on learning forms and filling them with meaningful values. The paper
gives an insight into the various Hidden Web crawlers developed for the purpose
giving a mention to the advantages and shortcoming of the techniques employed
in each.
