We've all received a computer file we didn't know how to open. The secret to identifying a compatible software program lies in the three-or four-letter extension at the end of the file name — which, for many of us, is a meaningless alphabet soup. This glossary will help you unscramble those mystery letters.

? AVI: Audio Video Interleave. Most often played on Apple QuickTime or Windows Media Player, this format for sound and video clips is becoming obsolete.