"DSL" stands for "Digital Subscriber Line", and resides on the same line as your telephone. However, rather than Dial-up, the DSL is on a different frequency than your phone line, so you can be online and talk on the phone at the same time. When you sign up for DSL, you will be sent a DSL modem, that plugs into the wall using a phone lines, as well as DSL filters, which need to be installed on every phone jack in the house, except for the one that the modem is plugged in to. For more information, see the Howstuffworks page on How DSL Works.