Sockets are what limit which type of CPU can go in.
There is the AM3+ for AMD mainstream and FM2 for their APU's. Intel has the 1155(just got last genned)/1150(latest gen) for their mainstream/enthusiast and the 2011 for the enthusiasts.
I would pick the AM3+ FX6300 if you're on a budget (Or PII X 4 965 if even more budget) and an i5 4670k on the 1150 if you've got the cash. If you're not into overclocking, get an 4570 or 4670.