KMS - Kernel ModeSetting = the way the correct resolution is set up for specific screens. (when you change resolution, this handles it at hardware level)
Mesa - the (only) OSS implementation of OpenGL.
Gallium - a sort of layer between graphic cards and a graphics API. The graphic driver only has to 'support' gallium3D.
DRI - how software communicates with the hardware.
But do read the links posted by chithanh.
A comparison between open source AMD and catalyst might be interesting. Their open source drivers aren't that slow to begin with. They might beat the blob, when performance doubles.
D3D9 for Gallium sounds really nice. Knowing the number of awesome games based on it, losing them would be a huge loss.