Asymmetric digital subscriber line (ADSL)
: is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide.
ADSL differs from the less common symmetric digital subscriber line (SDSL).
Bandwidth (and bit rate) is greater toward the customer premises (known as downstream) than the reverse (known as upstream).
This is why it is called asymmetric.
Providers usually market ADSL as a service for consumers to receive Internet access in a relatively passive mode: able to use the higher speed direction for the download from the Internet but not needing to run servers that would require high speed in the other direction.
When you might see it: Most people will come across ADSL on there Broadband router.
Typically a green ADSL light on a router means that the router has a connection to an exchange, the first step to ensuring a valid broadband connection.