the "socket" is the format for the pins in the bottom of the chip and the part on the motherboard that accepts the CPU, and it often also specifies the voltage. The number indicates the number of pins (usually, there is "socket A"). You can't plug a Socket X into a Socket Y, unless you have a converter, and those are rather rare. The pinout on the chip and motherboard can be important if you're thinking you may upgrade your CPU. It's also possible these days for chips to vary based on socket in subtle ways (more/less cache, slightly different frequency, bus speed, etc).