What does a video card do?
Allows for the rendering and display of 3d and 2d graphics, video capture and output to TV.
What does RAM do?
Random access memory (RAM) allows the computer to temporarily store data. Is volatile.
What does a Network card do?
A network card listens and monitors the traffic passing through the attached network. When it recognises a data packet addressed to it's IP address (or other network protocol identifier in use) it will accept the packet and pass it on to the PC attached.
What does a Hard disk drive do?
Stores non-volatile data, which is done by magnetism and an actuator arm that 'writes' data.
What does a sound card do?
It produces 3-D audio for games or surround sound playback for DVDs. It can also capture and record sound from external sources.
What does a Heat sink do?
It draws heat away fom the object that is is being used on. Helps cooling fans cool.