Parts of a computer

CPU: stands for 'Central Processing Unit' and is the 'brain' of the computer. Most CPU's today are made by Intel and bear such names as 'Pentium', 'Pentium Pro' and 'Pentium II'. Older Intel CPU's include the 80486 and 80386 families. Other manufacturers also make CPU's: Motorola for the Macintosh, AMD and Cirrus for PC's and others. The 'speed' of a CPU's processing is measured in megahertz. The CPU is the place that holds info about the operating system (DOS or Windows, for example).
RAM: RAM is what you know as “Memory”, as in how much memory does your computer have? It is not permanent memory - the RAM is erased when the computer turns off. Permanent memory is stored on the hard drive. Memory is measured in increments of bits and bytes. Generally the least memory you should ever have with a Pentium computer is 64 MB (megabytes: look up kilo-, mega-, and giga- for more info), and more is much better. There are places on the motherboard (called “slots”) for memory modules. The memory modules are small printed circuit boards with memory chips on them and are usually either SIMM’s (Single Inline Memory Modules) or DIMM’s (Dual Inline Memory Module). Don't confuse this with ROM.
This is an add-in  Expansion board such as a video card, sound card or modem. On every motherboard there are places to add circuit boards to extend the capabilities of the computer. The most common circuit boards used are the internal modem,  sound card, and the video display adapter. There are various types of expansion slots that may be on the motherboard. The ISA (Industry Standards Association) expansion slot is the older type and most of the older circuit boards used this type of slot. A more sophisticated type of slot is the PCI and the newer modems, and more sophisticated sound cards require this type of slot. The newest type slot in a PC is the AGP (Accelerated Graphics Port) slot, which is specifically designed for a video display adapter. The AGP port enables high performance graphics capabilities, especially for 3D graphics. The video card controls what you see on the monitor. It determines how many dots across the screen and down the screen the computer can look after. The more dots, the more information or the more detail you can see. The video card also controls how many colors you can see. Most computers today can show anywhere from 256 colors to many millions of colors. The sound card controls the sound. Most computers come with pretty cheap speakers, and the sound card can probably produce much better sound than the speakers can. With good speakers, today's sound cards can make your computer sound as good as a stereo!
Hard drive uses disks that are made of aluminum or glass (and therefore 'hard'). Each disk can store much more information than either a floppy or CD-ROM. Sometimes, there may be several disks in a hard drive. However, the disks in a normal hard drive can not be removed or replaced. Today, hard drives are measured in gigabytes. That's one thousand million bytes. 1 gigabyte is about 11/3 CD-ROM disks. Sometimes a special cache is used for quick retrieval of often-used information (such as web pages). This is just a separate directory on the hard drive.
Generally when we talk about a floppy disk drive we are talking about the drive that uses the 3.5 inch 1.44Mb floppy disk in it. There have been other types that have  come and gone.
CD-ROM stands for Compact Disk – Read Only Memory. The original name was WORM drive, which meant Write Once Read Many. So the term CD-ROM is not really very accurate, but it is the name that has stuck.
At one time there was the PC keyboard, the AT keyboard, and the 101 key enhanced keyboard, which had F9 through F12 keys and a separate numeric keypad. Now the 101 key enhanced keyboard is the standard type and keyboards are named according to the type of connection it makes to the computer. The two common types of connectors that go from the keyboard to the computer motherboard are the AT and the PS/2.  The AT is the larger older type, and the PS/2 is a newer type and communicates better with he computer.
There are a lot of varieties of mice in use today. Some have two buttons, some have 3 buttons, and some have a roller ball on top and  don't require you to actually move the mouse on the desktop. There are also programmable mice, touch pads that substitute for a mouse, etc.. The standard types are the Serial, and the PS/2. The Serial mouse plugs into one of the 9 pin serial port of your computer (COM1),  and the PS/2 mouse plugs into a special PS/2 port on your computer.
Modem allows your computer to connect to another computer using the normal telephone line. It converts data from a computer format, which requires many wires, into a format that can be sent using only the two wires of a telephone line. At the other end of the telephone wires the process is reversed. Data transfer rates from the modem vary from 14.4Kbs to 56Kbs. (14.4Kbs, 28.8Kbs, 33.6Kbs, 56Kbs) There are special types of modems such as cable modems that can communicate at much higher data rates.
USB (Universal Serial Bus) connector is the newest type port (connector) on the PC. It is extremely easy to use. You just plug in a USB compatible device and the computer automatically configures itself to use the device. The computer does not have to be turned off or rebooted. As many as 127 USB peripherals can be plugged into a computer at one time. At 12Mbits per second it is more than 100 times as fast as a serial port. The next generation USB motherboards will communicate at 480Mbits per second.                                                                   - http://stutzfamily.com/mrstutz/eisgeneral/computerparts.html