Digital Subscriber Line. These lines carry data at high speeds over standard copper telephone wires. With DSL, data can be delivered at a rate of 1.5 megabits per second (or around 30 times faster than through a 56K modem). In addition, DSL users can receive voice and data simultaneously, so they can use the Internet and make phone calls on the same line at the same time.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf