The CPU is a very powerful thing, but all it does is work with information. It needs to get this information from other kinds of devices. Any component of a computer system that sends information into the CPU is called an input device.
Memory is like the banks of switches we have been thinking about that simply contain 1/0 binary patterns. Read Only Memory (abbreviated ROM) is a special kind of memory that cannot be changed. The most basic instructions for the CPU are built in to the ROM at the factory. To the end user, there really is very little to worry about regarding ROM. The amount of ROM in your computer doesn't really matter to you. All your programs and information will go into another kind of memory that we will learn about soon.
The keyboard is probably the easiest kind of input device to understand. When the user presses a key, a code is sent to the cpu, which translates the code into some sort of storage format (usually ASCII.) There is generally no way for the cpu to send information back to the keyboard, so it is an input only device. Usually, you buy a keyboard as part of a computer system, but there may be a reason you want a specialized model. Certain keyboards are designed to be more ergonomically safe to prevent Carpal-Tunnel Syndrome (a common malady of those who spend all day at keyboards.)
Many modern computers rely heavily on the mouse. This is a small object, usually with a roller ball on the bottom, that can be dragged along the desktop. Mice usually have one or more buttons on them that can be activated with the hand controlling the mouse. As the mouse is moved on the desk, a pointer moves on the screen. This mouse pointer is analogous to the user's hand. Many find this a more natural way of controlling the computer than typing on a keyboard. There are a number of "mouse substitutes" available, but they all basically work in the same way. Laptop computers often have a small ball embedded in the keyboard that can be rolled so the mouse can be activated even when the computer is being held on the user's lap.
The same kinds of technology that enable us to store music digitally on compact disks allows us to store other kinds of information on the same medium. Compact Disks essentially store numbers. They are inexpensive to create, and can hold large amounts of information. (600 Mb) CD-ROMs are frequently used to sell software which has become too large to fit on floppy disks. Unfortunately, a CD - ROM cannot be written to with typical home technology. The ROM part of CD-ROM refers to this characteristic. A CD - ROM is an input device because it can send information to the CPU, but the CPU cannot send information to it.
CD Drives are measured in comparison to the speed of music CDs. An audio CD always runs at a constant speed. A 2X CD drive is twice the speed of an audio CD player. 6X and 8x drives are available at this writing. Even these drives are not as fast as typical hard drives.
Erasable CD drives are becoming popular, because they hold as much information as a regular CD, but the user can store things to them as well as reading from them. Such devices will probably be much more prevelant as their price comes down and reliability improves.
There are other devices available for input, but they tend to be specialty devices. Some examples are: