A sound card is what you plug your PC speakers into. This card allows you hear music from your CD-ROM drive, and PC sounds that your software generates. These are a must for video games.

A modem, allows you to connect your PC to your telephone line so you can connect to the Internet or other online services.

I/O cards are what you plug your printer or your mouse. However, these cards are more common in older PCs as most new PCs have these built right into the motherboard. These newer motherboards, called integrated motherboards, free up more expansion slots by having this circuitry built in.