In addition to the CPU, motherboard, and hard drives, a computer generally has several expansion cards, which provide it with video, sound, and other miscellaneous functions. These cards commonly fit into one of two kinds of expansion slots on the motherboard, PCI and ISA.
PCI is the current standard. PCI is used for almost all sound cards and I/O cards, and some videocards. PCI slots are generally white or off-white, and located so as to allow the external ports/adaptors of the cards to face out the back of the case when installed.

ISA is an older standard, and is often found in elderly (more than 3 years old) systems, sometimes even as the only type of slot available. It is slower than PCI, and ISA cards are no longer being produced, although they can generally be found in used computer parts shops or at computer fairs. ISA slots are generally black, longer than PCI, and, on a motherboard with both PCI and ISA slots, will generally be beneath them.

AGP is another standard slot type, specifically designed for videocards. It passes information at faster speeds, and thus is especially suited for graphical hardware. The AGP slot is generally above all the other slots on the motherboard, is farther from the back of the case than PCI or ISA slots, smaller, and is colored brown.

There are many good reasons to upgrade your expansion cards. From adding networking capability, to allowing you to use your computer to record TV shows, expansion cards allow you to expand and increase its functions exponentially, generally at a reasonable cost to the user. And the vast number of different PCI and ISA cards out there insures that if you want to do something with your computer, the means to do it is probably out there somewhere.

Video Card: A video card is the link between your computer and monitor, Video card quality is determined mostly by the amount of memory on the card, and by the quality and speed of the 3-D accelerator, often referred to as a Graphics Processing Unit, or GPU. The information on both will probably be available on the box the product came in, the manual, or the company's website.

Sound Card: A sound card allows you to use your computer to play music and other audio data from your computer. Sound cards also have onboard memory, but the main criterion for sound cards now has become their compliance with the 5.2 surround sound standard. With a 5.2 enabled sound card, you can use your computer as a surround sound decoder.

Capture Card: A capture card allows you to view and record video from sources such as television, VCR, video games systems, and camcorders on your computer. A great idea for situations where having both a computer and a television would be wasteful of space.

SCSI/RAID cards: Besides the IDE standard, there is another standard for drives, the SCSI standard, While marginally faster than IDE, SCSI offers few advantages, and is vastly more expensive and complicated. RAID, a system using multiple SCSI drives as a single unit to improve speed and reliability, is very effective, but very costly, and RAID capability has recently become more common with IDE baed drives.

USB/Serial/Parallel Port Cards: If all your USB/Serial/Parallel ports are being used, you can install a card with additional ports, thus expanding the capability of your computer to use external devices.