DSL¹ - an acronym meaning "Digital Subscriber Lines" - provides high-speed Internet access over
your existing telephone wiring2. Data transmission takes place using hardware attached to both
ends of the telephone line, utilizing frequencies above the normal telephone bandwidth
(from 300Hz to 3,200Hz). The speeds achieved are much faster than those you can get by modem
over so-called "plain old telephone service" (a.k.a. POTS).
The connection is constantly on (a.k.a. 24/7) so you don't have to "dial in" to connect.
Connection speeds vary with distance from the telco's Central Office (CO). The closer you are,
the faster the service, and vice versa. Bandwidths for ADSL, the most common DSL, are
typically in the 150Kbps (K/second) range for download, and 10Kbps for upload (again, this works
best if you're close to the CO). Farther from the CO, download speeds may be as low as 32Kbps.
DSL is still under development, and speed improvements are inevitable.
A word about ADSL: There are two main reasons for the Asymmetric DSL implementation. First,
the technical reason: Due to the high density of wire terminations at a CO,
there can be a lot of interference between lines, called crosstalk in telephony circles.
The second reason is that an asymmetric design mirrors actual usage - there is considerably
more download than upload for most users.
With some DSL service, a "POTS splitter" is required. The splitter is an unpowered device
that insures that the traditional POTS voice service is available on the phone line. Note
that with DSL your normal phone line is available for voice use - callers do not get a busy
signal.
Note that certain packet-based applications, such as two-way video conferencing, do not work
very well over DSL. DSL is ideal for most gaming applications, however.
To hook up to a DSL service, you will usually need a 10Base-T adapter which connects to
the external DSL device. The telco will usually provide, and even install, this adapter.
The cost of DSL is roughly equivalent to that for a second phone line and dial-up Internet
access. (Note that DSL does not require a second line.)
Risks: If you have file sharing enabled, data on your hard disk is potentially at risk. You
can protect yourself by using a firewall (on another PC or with a hardware device), or at minimum you can disable file sharing if you have a standalone computer. For step-by-step instructions on this, see:
http://www.ehow.com/eHow/eHow/0,1053,13617,FF.html?src=bre4. Note that even then you are not safe, see cable modem by Blue_Bellied_Lizard for more on security with a persistent connection.
For more details about DSL, see http://homepage.interaccess.com/~jkristof/xdsl-faq.txt, which
was the primary research source for much of the forgoing content.
1. Includes xDSL variants such as ADSL (Asymmetric DSL) and HDSL (common in Europe). See Types of xDSL.
2. This is sometimes referred to as copper pair or twisted-pair wiring.
Noded by request