A T-1 is a digital line that has a data capacity of 1.544 million bits per second. (mbps)
Cable and DSL's are usually hooked into T-1's (or t-2,3,4) at your provider.
T-1 lines have been popular for business for a long time and have better up time than cable or dsl technologies because T-1's have been around for longer, and telco's are well versed in fixing/maintaining them. When they do go down, they are usually up very quickly.
Cable sometimes has higher bandwidth, but a T-1's ping time is always better (more responsive) than cable or dsl.