What is the difference between ADSL and DSL? Also is there any difference in the model types and connection sources (cable vs telephone lines)? Why is ADSL cheaper from other ISP than DSL from Bell Sympatico?

ADSL is a subset of the DSL family of services/technologies.

Learn more here: Howstuffworks "How DSL Works"

Most "DSL" lines in the US are actuallty ADSL. See:

http://en.wikipedia.org/wiki/Digital_subscriber_line