Materials

The majority of T1 lines are made out of twisted strands of copper, covered by a layer of protective plastic. However, there are some T1 lines that use fiber optic cables instead of copper, which produces slightly faster connection speeds.

Function

T1 lines are used to transmit voice and data. They function similarly to phone modem cables, but they have much higher data transfer speeds. For example, a phone modem cable has a data transfer speed of 30 kilobits per second (kbps), whereas a T1 line has a data transfer speed of 1,544 kbps.

Cost

T1 lines are expensive because of their high connection speeds. It is impractical to purchase a T1 line for basic household connections, such as Internet, phone service, or cable TV.

Real World Uses

Small businesses use T1 lines to handle all their phone and Internet needs. Larger businesses may use multiple T1 lines. Telephone service providers use T1 lines to transfer calls.

Other T lines

There are T2 and T3 lines, which are upgrades of the T1 line. For example, a T3 line can transmit data 28 times faster than a T1 line. There are "T" connections that are even faster than the T3 connection, but they are not in use. The EU (European Union) uses a very similar connection called "E", instead of the "T" connection; these two connections are not compatible with each other.

Source:

WiseGeek: What is a T1 line?