What does ADSL stand for?
Asymmetric Digital Subscriber Line.
 

What is ADSL?
ADSL is a broadband connection technology which utilises existing copper wired telephone networks. ADSL uses a wide range of frequencies over an existing telephone line to deliver much higher speeds than your standard 56k modem (between 10 and 40 times faster). It is also possible to use your phone while online.

How does ADSL work?
DSL utilizes more of the bandwidth on copper phone lines than what is currently used for plain old telephone service (POTS). By utilizing frequencies above the telephone bandwidth (300Hz to 3,200Hz), DSL can encode more data to achieve higher data rates than would otherwise be possible in the restricted frequency range of a POTS network. In order to utilize the frequencies above the voice audio spectrum, DSL equipment must be installed on both ends and the copper wire in between must be able to sustain the higher frequencies for the entire route. This means that bandwidth limiting devices such as loading coils must be removed or avoided.

ADSL Asymmetric Digital Subscriber Line
(ADSL) is the most popular form of xDSL technology. The key to ADSL is that the upstream and downstream bandwidth is asymmetric, or uneven. In practice, the bandwidth from the provider to the user (downstream) will be the higher speed path. This is in part due to the limitation of the telephone cabling system and the desire to accommodate the typical Internet usage pattern where the majority of data is being sent to the user (programs, graphics, sounds and video) with minimal upload capacity required (keystrokes and mouse clicks). Downstream speeds typically range from 144 Kb/s to 2.3 Mb/s Upstream speeds typically range from 96Kb/s to 2.3Mb/s

What does a splitter do and when do I need one?
A splitter uses a low pass filter to separate the low end frequencies of the telephone audio spectrum from the higher frequencies of the xDSL signals. The splitter should be a passive device, not requiring power so that "life-line" voice service can be provided as has been in the past. This splitter allows for the traditional voice service that consumers are accustomed to. A splitter is required at both the customer premises and at the far end (CO). xDSL that does not use a POTS splitter on customer premises is termed "splitter-less xDSL". However, there really is no such thing as splitter-less xDSL. The splitter function in these cases is just performed at the provider, generally the CO. Whether a POTS splitter is required or not depends on the xDSL service being provided.