Perian is a Quicktime component (similar to Flip4Mac) that enables you to decode a number of formats without using standalone applications like VideoLAN.
It allows you to playback DivX, XviD, FLV and AVI - i.e., it adds support for other common AVI codecs (Quicktime already has partial support), letting you play MS-MPEG4 v1, MS-MPEG4 v2, MS-MPEG4 v3, DivX 3.11 alpha, 3ivX, Sorenson H.263, Flash Video and Truemotion VP6.
Besides those variations, it also lets you play these formats from an AVI envelope: H.264, MPEG4, AAC, AC3 Audio, and VBR MP3.
With versions prior to 1.0, you also needed to install A52Codec to play files with 5.1 AC3 audio (this is not required anymore, since it is now shipped with Perian).
But let's not kid ourselves - most people will want to use this with Front Row...