MAC is a file extension for a macro file format used by many programs. A macro file stores a sequence of commands for quick and often repeated functions. When the user types a keyboard shortcut for a particular macro, the tasks programmed into that macro are conducted.
Many applications have built-in macros and many also allow users to create their own.
Learn more about MAC files:
Read about macro viruses at Search Security.
Find answers about macros at IT Knowledge Exchange.
See our Browser Shortcut Cheat Sheets.
About.com explains working with macros in Word.