You n00b
As previously stated, a T1 is 1MB down 1MB up. Quite expensive too. The new connection at my house is faster then a T1 download speed wise, and costs nowhere near as much. T1's are used in small businesses mainly
DSL - Uses the phone line to connect, speeds vary depending on the provider. Bandwith is capped by the provider, not shared bandwith like cable is. Verizon is one of the big DSL providers.
ISDN - Basically 2 56K modems working together and is 128kbps. Not fast at all and somewhat costly (last I looked anyways)