From the phone switch, the data travels through your phone line. This path is actually fairly secure. The data for all DSL users does get carried over the US West high-speed network, but only US West employees should have direct access to that data. Since the phone line running from the phone switch to your home isn't used by anyone else, no other DSL users can see the information intended for your computer. Cable Modems Cable modems work similar to the diagram below.
Data comes in from the Internet to the cable company's data network.
All the homes connected to that smaller network get all the information intended for the other homes in the area. Again, this isn't a problem as far as using the service on a basic level. Your computer would ignore anything not sent directly to it. However, someone in the neighborhood could run a special program and see the information sent to everyone in the same area.