OK, the first answer certainly is fancy, but if you just want a list of the various pieces in your system, here it is:
processor - actually does the computer's computations
motherboard - organizes and connects
hard drive - stores data
memory/RAM - where it keeps info being currently accessed
cd/dvd drive - obvious - where you put the cd or dvd in
sound card - sometimes part of the motherboard, sometimes
added in. this is a processor dedicated to sound
video card - same as sound card, but for video (duh)
modem - connects to a phone line to get to internet
Ethernet - connects to DSL or Cable
fans - keeps system cool. important for computer's health
various wires - connect all components.
These are the basics. There are other things that can be added - floppy drives, card readers, video capture cards, physics processor cards, extra fans, LCD displays, even cupholders.
I hope this helps. contact me if you have more detailed questions about these components or anything...