The motherboard is probably the most importart part of a computer. It links all the input/output devices and processing devices together. The motherboard is the big board you see when you look into the computer (you can't miss it). It will have several fans on it and a bunch of cards sitting at 90degrees to the board. The motherboard connects the graphics card, hard drives, cpu, memory, sound card, floppy drives together etc.
The pins and sockets you talk about are used to connect the cpu to the motherboard. Socket 939 is the old AMD64 platform and literally has 939 pins. Now AMD use a socket called AM2, but it's still really new. Intel are currently on socket 775 if I'm right. Each different series of cpu will generall have it's own socket. For example, you cannot plug an Intel chip into an AMD motherboard.
Socket 1024 is a server socket, I think. Empthasis on think!
Hope this helps :S