A file is a container that holds information. Most of the files you use contain information (data) in some particular format--a document, a spreadsheet, a chart. The format is the particular way the data is arranged inside the file. The format of a file is known as its data type.
When File Manager is in one of its icon-view modes, you can identify the data type of a file by the icon used to represent the file. Each data type has a different icon.
Most application programs understand a limited number of data types. For example, a document editor probably cannot read a spreadsheet file. The desktop helps you recognize different types of files using a data-type database. In most cases, when you double-click a file, the desktop will automatically launch the application that understands that file's data type.
The maximum allowable length of a file name varies from system to system. Some operating systems do not allow file names longer than 14 characters. If necessary, consult with your system administrator.