The physical socket on the motherboard that the CPU plugs in to. Different CPU families need different sockets, all incompatible shapes and sizes.
| Socket 462 (aka Socket A) | AMD Duron and Athlon | Socket 754 | AMD Athlon 64 | Socket 940 | AMD Athlon Opteron | Slot 1 | Intel Pentium II, III and Celeron (233-650MHz?) | Socket 7 | Intel Pentium | Socket 370 | Pentium III and Celeron ?
Note - Socket 370 to Slot 1 adapters exist
One page links to CPUSockets: