DSL, or Digital Subscriber Line, is one method of broadband access to the internet. The term is actually a shorter version of the more correct ADSL, or Asymmetric Digital Subscriber Line, the most common type of broadband access in the UK.
One very large advantage of ADSL is that the service can be delivered along existing telephone lines. This means no re-wiring of a room or house, and very little installation of extra equipment. In fact, a homeowner can usually do the minimal installation required, as it just involves clipping filters into existing phone points.
Filters are needed because, although ADSL uses the existing phone lines, the digital data is transferred at a different frequency than the voice data. Without filters, interference would degrade both types of data. Phone calls would get dropped or have a lot of static, and data might never get to the internet.
The designation of Asymmetric is due to the fact that data coming into the customer typically transfers at a higher rate than data leaving the customer and heading toward the internet. Using a Symmetric Digital Subscriber Line, or SDSL can solve this, but the monthly cost of this type of line is typically higher, and it’s rare for a residential customer to have this type of service.
DSL broadband services also require a DSL modem to convert the voltage signal from the phone line into a digital signal that the computer can use. However, in many cases when you sign up for ADSL broadband services, the service provider will supply the modem for free or at a very low cost. If you change providers, it typically has to be returned. Read the fine print in your subscription contract or ask a salesperson about the requirements before you commit to anything.