This is some general info from DSLEXTREME.com
A T1 or DS1 line is a private, dedicated line that goes directly from your office to your Internet Service Provider (ISP), providing transmission at 1.544Mbps, roughly 60 times more data than a normal residential modem. It is also extremely reliable, much more reliable than an analog modem. This speed is guaranteed at any distance, but the farther you are the more expensive the T1. Generally speaking, the cost of a T1 ranges about $400 - $800.
Depending on what they are doing, a T1 line can generally handle quite a few people. For general browsing, hundreds of users are easily able to share a T1 line comfortably.
Unlike a DSL connection, a T1's dedicated connection provides guaranteed bandwidth, since the line is not shared with other users. Unlike the requirements needed to get DSL, anyone can get a T1. The T1 / DS1 standard is used in the North American and Japanese marketplaces.