A modem can help you get in touch with the world, whether you're curious about the origin of hot dogs, interested in learning more about a prospective employer or simply want to stay in touch with your best friend 2,000 miles away.

Modems are much like a telephone for your computer, letting your system communicate with other computers around the world. Modems accomplish this by providing a connection between your computer and a regular phone line. They translate the digital signals from your computer into analog signals that can be transmitted over the phone. (The "screeching" you hear from a modem is the sound of these analog signals.)

The name "modem" -- an abbreviation of "modulate/demodulate" -- refers to the process of converting digital information to analog and then back again. When your modem connects to another modem and computer that are set up to receive incoming calls, a host of information and services is just keystrokes away.

Some characteristics that distinguish one type of modem from another include:

Transmission speed. Transmission speed, the rate at which a modem can exchange data with another modem, is measured in bits per second (bps), or kilobits per second (Kbps). Old-timers may also refer to the data transfer speed as the "baud rate."

Most modems sold today advertise maximum speeds of 33,600 bps or 56,000 bps, but they won't move data quite that fast unless your phone lines are excellent and your equipment perfectly matches the modem at the other end. In fact, the maximum transmission speed is dependent on the top speed that both modems can support: If you have a 33,600 bps modem, but you're connecting to an old 2,400 bps modem, you'll only be able to communicate at 2,400 bps. When two modems connect, they usually "negotiate" a mutually acceptable speed.

All these zeros frequently are left out of advertisements. So you'll see a 56,000 bps modem advertised as a "56K" model.

Data compression capabilities. Information being sent out over a modem can be compacted by using compression, which allows more data to be sent at once. This can boost the effective speed of a modem well beyond its official transmission speed. Compression methods supported by many modems include MNP 5, for compression up to two times, and V.42bis, for compression up to four times. Compression is a function of the modem, not the software, and the modems on both ends of the phone line must support the compression scheme for it to work.

Fax capabilities. Newer modems can send and receive faxes as well as data. With the proper software, you can treat a fax modem as a printer. When you "print" to a fax modem, it sends your document to a fax machine or fax modem somewhere else. Because of international standards, fax transmissions are slower than data connections, with a maximum speed of 14.4 Kbps. So you'll frequently see modem speeds advertised as 56.8/33.6/14.4 fax.

Speakerphone capabilities. The fanciest modems throw in regular telephone circuitry with their data and fax capabilities. With a microphone and speakers, you can use these modems as speakerphones. Some external speakerphone modems have microphones built right into the case.

External vs. internal design. Modems come in two flavors. External modems are encased in small boxes which sit on your desk and connect to your computer's serial port with a cable. Internal modems are circuit boards that fit into an expansion slot inside your computer. You plug your phone into a jack in the back of the PC. Internal modems are generally cheaper, while external modems are easier to transfer to another computer. Internal modems draw electricity from the computer's main power supply, while external modems must be plugged in separately. External modems also have flashing lights that make it easier to monitor your transmission.

Error control capabilities. Modems use a variety of error-control methods to confirm that no information was lost in the transfer from computer to computer. This is especially important for high-speed connections. Some common error-control protocols include MNP 2-4 (2 through 4) and V.42, which incorporates MNP 2-4. Neither of these schemes is considered better than the other, but MNP is the de facto standard. While the modem software enables the user to select the error correction, the actual task belongs to the modem itself.