DSL is an acronym for Digital Subscriber Line, there are several different types of DSL, which are commonly collectively referred to as xDSL (where x represents all the DSL variants).
xDSL services are characteristically potentially always-on (connected) services.
xDSL services are categorised as being Broadband services due to the considerably higher data transfer speeds that can be achieved compared to Narrowband services such as dialup.
ADSL is an acronym for Asymmetric Digital Subscriber Line, where Asymmetric refers to a difference between the achievable download and upload speeds, and should not be confused with the term Asynchronous - note: ADSL is not Asynchronous.
ADSL is also a Synchronous technology, which means that uploading and downloading of data takes place simultaneously using separate frequency bands within the overall ADSL frequency band.
ADSL is deployed using two ordinary copper telephone wires, and can comfortably co-exist with analogue voice services sharing the same two copper wires, where ADSL operates on a much higher frequency (same copper wires) so as to avoid clashing with analogue voice services that operate on a much lower frequency range (same copper wires).
Joe Leichleder is the person that came up with the idea of ADSL in 1987 while working as a researcher for Bellcore.
Initially ADSL was seen as a potential Video on Demand competitor for cable TV services, it was only later that ADSL became an important means of facilitating internet connectivity.
Telecoms companies and other hardware vendors got involved in developing and rolling out ADSL circa 1989, and several refinements to the initial ADSL standard were made - as well as divergent ADSL standards and modulation schemes.
ADSL is limited by the length of the two copper wires it uses (also known as copper distance): the longer the two copper wires are, the more noise present on the line, which consequently drastically reduces achievable data transfer speeds. Most ADSL modems and routers are capable of measuring noise on the line, which is usually referred to as a Signal to Noise Ratio (SNR).
ADSL works best where the copper wire length is less than 5km.
Copper quality also affects ADSL: ADSL does not work well over copper wires that are old and corroded.