Asynchronous Digital Subscriber Line is a service provider technology that enables a far more efficient transmission of data across a fixed line copper telephone connection. ADSL allows far more data to be carried across the legacy copper infrastructure that for example the modem technology of the nineties.
Furthermore, ADSL permits voice and data to co-exist over the same physical copper wire. It manages this by using a micro-filter to separate the high frequency data transmissions from the low frequency voice, this makes both easily discernible and easily isolated. The A in ADSL relates to asymmetrical, as download speed (bit rate and throughput) is larger than the upload performance, which is in accordance with typical residential consumer internet behavior.
ADSL however has not just been restricted to residential customers but has also found acceptance within SMB and the enterprise. New initiatives such as software defined wide access networks has embraced ADSL as a means for bundling cheap ADSL links to gain larger capacity or for determining cheaper or more reliable routes.