A digital subscriber line (DSL) is a high-speed Internet connection that works through your telephone line. It's different from a dial-up modem in that the phone line doesn't have to commit to either the phone or the Internet: you can use both at the same time. DSL is also much faster and more reliable than a dial-up modem.

Internet data can run through standard copper-wire phone lines because these lines have always been able to handle a much greater data load than a simple phone call. In terms of raw speed, DSL is not quite as fast as a cable modem, but in practical terms they're about equally efficient.

Human voices are carried over phone lines within a very small range of frequencies, usually up to about 3,400 Hertz. But a phone line can handle frequencies up to several million Hertz, and it's at these higher frequencies that Internet data travels.

An asymmetrical DSL system (ADSL) is set up on the assumption that users will download more data than they upload, so the connection speed will be much faster if you're receiving data than if you're sending it. Exactly how fast DSL works will depend on your distance from the provider's central office.

If you have ADSL service, you'll have a DSL transceiver (modem) at your location. It will usually connect to your equipment via USB or 10 base-T Ethernet connections. A larger business might require more complex equipment.

One advantage that DSL has over cable service is that with DSL, your service doesn't lose efficiency as quickly as cable does, if a lot of people in your neighborhood are online at once.