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).