Acronym for Central Processing Unit.
The bit of the computer that does all the execution of commands.
There are several different arch's that Linux supports, including (but not exclusively):
Other CPU families include:
CPUs come in 2 flavours: RISC and CISC, although the lines between the 2 are not clear-cut as they should be in theory...
See CPUSockets for cpu fittings onto motherboards, and MotherboardMonitoring for checking the current temperature of your CPU (under linux).
96 pages link to CPU: