DSL is an acronym for Digital Subscriber Line, a technology that transmits digital data across existing voice telephone lines.
DSL is more correctly referred to as ADSL in most cases. The “A” stands for asymmetric, which means that data is transmitted faster in one direction than the other. In most cases, that means that the download speed of an ADSL connection is faster than the upload speed. This tradeoff reflects the fact that in normal internet usage, people download significantly more than they upload.
DSL coexists with voice (a.k.a. POTS) on the same wires by being transmitted at frequencies above human hearing.