brockenspectre,
DSL is short for Digital Subscriber Line. It is a much higher speed connection that using a standard modem. NTL owns the lines which the service is carried over. AOL is a reseller and they are selling you the service that AOL bought from NTL at a discounted bulk rate. You will be billed though AOL for the service that is provided by NTL.
With DSL it is an "always on" service. In order to use a Fax though your computer, you will still need the modem in your computer. The frequency that DSL operates on your phone line is different from the one a standard modem uses and is out of the audible range.
The filters keep any of the DSL signal from bleeding though and it protects any standard modems from being damaged by the DSL signal.
Take Care,
Richard