DSL is an abbreviation for Digital Subscriber Line. It simply means that you are getting internet over the copper phone lines running to your home, rather than over a cable TV line, fibre, or other means.
The quality and speed of DSL internet varies with the distance from the home to certain equipment on the network, as well as other factors. In some cases this may make the speed very low, or even unusable. If you currently have DSL, switching DSL internet service providers is unlikely to solve this problem.
