DAT is simply a file that can contain any type of data, usually specific to the program that created it. You might be able to read it using a program like Notepad, but the best way is to open it via the program that wrote it to begin with.
dat files are associated with different programs, so the answer is not always the same program. A good idea however, is to open it with notepad as you can usually recognize some of the information.
A Dat file is a type of file that belongs to no program in specific. Many programs can open dat files, but may not read them properly and display the data. The best thing to do is to find out what program created the file, and open it with that.
Dat files store various binary data for different applications, including windows, there is no standard format, use a hex editor program like HxD to examine or edit them.
