The Modem is a hardware device that handles the communication between your computer and your ISP.
The quality of the modem is measured by the rate it can send and receive information. A faster modem means better performance when working on the Internet. For this reason, the faster the modem you can afford is usually the one you should buy. This rule is not always true. The rate of information is also determined by your ISP.
Another distinction between the Modems is whether they are located inside the computer (An Internal Modem) or outside (An external modem). An external is less expensive, nevertheless it is harder for installation and maintenance.
