different cpu's fit on different motherboards. they're identified by sockets.. so a socket 939 cpu will fit on a socket 939 motherboard. sometimes the number refers to the number of pins on the cpu.. newer generation of cpus will come out with newer sockets. for example, socket 754 didn't support dual channel memory, so the newer socket 939 had support for it... but the ram it used was DDR, the newer AM2 has support for DDR2.. and so on..