A .dat, also known as a DAT file, is a data file, according to About.com. DAT files contain data in text or binary formats. These files are commonly used in the popular Minecraft game.
MakeUseOf states that while DAT files can be found in many locations, they are most commonly seen as email attachments, and most commonly created by Microsoft Exchange Servers.
When trying to open a DAT file, MakeUseOf recommends clicking on the file and allowing the computer to select the program to open the file with, if any. If that fails, many DAT files can be opened using any text reader, though some can be more complicated and require the user to convert them into a different format.