Jasper St. Pierre has posted a lengthy overview of the Linux graphics stack. It's a good starting point for anybody who is not clear on what all those acronyms mean. "The X server needs to know what’s happening here, though, so it can do things like synchronization. This synchronization between your glxgears, the kernel, and the X server is called DRI, or more accurately, DRI2. 'DRI' stands for 'Direct Rendering Infrastructure', but it’s sort of a strange acronym. 'DRI' refers to both the project that glued mesa and Xorg together (introducing DRM and a bunch of the things I talk about in this article), as well as the DRI protocol and library. DRI 1 wasn’t really that good, so we threw it out and replaced it with DRI 2."