ADSL is the most common form of DSL used in the U.S. today, and HDSL is the fastest DSL available with speeds of up to 1.5 mbps. DSL stands for "digital subscriber line."
The type of DSL service offered depends on the Internet company and the location of the home or business.
HDSL stands for "high bit-rate DSL," and is generally used for businesses because it is fast enough to support teleconferencing and web hosting. It was one of the first forms of DSL to hit the market, and is an alternative to a T1 line.
ADSL stands for "asymmetric DSL." It uses a customer's existing phone line to provide Internet speeds of between 512 kbps to 6 mbps. ADSL can accommodate analog and digital information at the same time.
Besides ADSL and HDSL, there are a few other types of DSL. CDSL, trademarked by Rockwell Corporation, is slower than ADSL but doesn't require the customer to use a splitter for reliable service. RADSL is a technology that incorporates software to determine the rate at which data can be transferred over any telephone line and can adjust itself accordingly. VDSL stands for "very high digital subscriber line," and delivers speeds of between 6 mbps and 25 mbps, higher in some cases.