Stands for "Digital Subscriber Line." It is a medium for transferring data over regular phone lines and can be used to connect to the Internet. However, like a cable modem, a DSL circuit is much faster than a regular phone connection, even though the wires it uses are copper like a typical phone line.
An asymmetric DSL (ADSL) connection allows download speeds of up to about 1.5 megabits (not megabytes) per second, and upload speeds of 128 kilobits per second. That is why it is called ADSL and not just DSL (because of the asymmetric speeds). There is also a "Symmetric Digital Subscriber Line" (SDSL) which is similar to ADSL, but allows data transfer speeds of 384 Kilobits per second in both directions. Theoretically, this type of connection allows download speeds of up to 9 megabits per second and upload speeds of up to 640 kilobits per second. The difficult part in establishing an DSL circuit is that it must be configured to connect two specific locations, unlike a regular phone line or cable modem. DSL is often seen as the new, better alternative to the older ISDN standard.