Digital Subscriber Line (DSL) is a generic term categorizing services provided over copper wire. DSL subscribers may receive high speed Internet service and other services bundled with the DSL package. When a service provider or ISP offers Asymmetric Digital Subscriber Line (ADSL), the customer will receive service that is not guaranteed to be the same speed in both directions.

For example, many ISPs advertise their Internet service with a download and an upload speed, 1Mbps download and 256Kbps upload (or something similar). These advertised speeds are rarely the same in both directions. This would be the way that ADSL works, the upload and download speeds offered by the ISP to subscribers is different and stated that way.

DSL is more generic, implying any type of Digital Subscriber Line service, from ADSL where the upload and download speeds are different, to symmetric digital subscriber line (SDSL) in which the upload and download speeds are the same. Service providers are introducing new methods of ADSL style technologies, including tiered pricing which allows the ISP to charge higher fees for more bandwidth for either downloading, uploading, or both. ADSL is a specific type of service sold to allow subscribers to connect to high speed data networks. The majority of the DSL service sold for residential access is ADSL.