Expansion Slot: Internal PCI, AGP, or ISA slots on the Mainboard to plug peripheral cards like modems, LAN cards, video cards, sound cards, game cards, TV and radio cards, and many other devices.
Drive Bay: Internal drive bays are to mount a hard drive. External drive bays are accessible from the exterior of the case for items like: Floppy drives, LS120 drives, Zip Drives, CD ROMís, DVD CD ROMís, Re-Writable CD ROMís, Tape Backups, etc.
Mainboard: The largest main circuit board in the computer. Also known as the "motherboard". All other components are attached to this board and is probably considered the most important piece of the computer. The Mainboard includes the main chipset, cache, expansion slots, HDD/FDD controllers, and several other critical items.
CPU: Central Processing Unit. Measured in "MHz" to calculate the speed. The higher the number the faster the computer runs (example: 750MHZ is faster than 500MHZ). Consider this component to be the engine or horsepower of the computer. Common brands are Intel, AMD, Cyrix and a few others. Common models are 486, Pentium, Pentium II, Pentium III, Celeron, K6, K6-2, Athlon, and many others.
RAM: Random Access Memory. Measured in MBís or MegaBytes. RAM is a temporary storage place for data retrieved from the slow mechanical Hard drive, then is quickly routed to the CPU for processing. Itís difficult to explain how RAM effects the computer in a few sentences. The best way is to use the automobile as a comparison. Think of RAM as the carburetor on a cars engine. It is easy to understand why a 4 barrel carburetor is faster than a 2 barrel, because a 4 barrel shoots more gas into the engine at once; therefore, making the car go faster. Very similar with RAM. 64MBís of RAM is faster than 32MBís because it shoots more data into the CPU at once; therefore, making the computer run faster.
Hard Drive: The hard drive is the main storage place for your Operating System (Windows), programs, files, pictures, etc. Many people compare it to a filing cabinet in your office, where you store paperwork and file folders. Measured in MBís (MegaBytes), or more currently GBís (GigaBytes). The size you need depends on how many programs and files you would like to store. Like your filing cabinet, it can hold so much paperwork before you need a larger one. Internal components include Heads and Disk Platters. The Disk Platter is just what it sounds like: a round disk. The Heads are similar to the head on your cassette player or VCR where it magnetically reads and writes the information on a tape, or in this case a Disk Platter. The Disk Platter spins at very high speeds, and the Heads float over the platter which mechanically moves to different parts of the disk to read and write the data. This is the most delicate component in the computer. For instance, if you move your computer and accidentally bump or drop it, the heads may hit and scratch the disk platter which will damage or corrupt your data for good! Be very careful when moving or transporting your computer.
Video: The Video card (or VGA card) converts computer data to an image the user can see in the form of print or graphics viewed on the VGA Monitor (your display screen). Todayís computers need fast video cards to process images for 3D games, graphical pictures, spreadsheets, etc. A poor video card will "draw" the image on the monitor slowly or choppy in games. This can create a critical performance bottleneck depending on the users software applications. Common high-end gaming cards are made by 3DFX Voodoo, nVidia and others.