The extension DAT is generally used for datafiles, ie data files. DAT files are nonspecific and can not be assigned to any data type. They can contain anything, graphics, audio, texts or simply data of the generating program.
Dat files are used and generated by quite a few programs and can normally only be used by these programs. You can use a text editor or rather a hex editor to easily view (look in) the file. However, these data views are usually useful only if there is a lot of prior knowledge on the part of the user.
Data files are for example used by Microsoft Visual Studio, Winamp, Windows and Firefox as well as many games.