The AGP connector is a slot on the motherboard into which you insert the computer's video card. It is not accessible from the outside of the computer.
The PS/2 connectors are the small round connectors on the motherboard into which you plug your mouse and keyboard. They are accessible from the outside of the computer.
The serial port (you might have two depending on your computer) is a D-shaped connector with nine pins. It is used for connecting old-style mice or external dial-up modems, and it is accessible from the outside of the computer.
There is no single connector known as a "socket". "Socket" is just a generic term referring to a connector.
I hope this helps!