A T1 is a leased line, a dedicated telephone connection which consists of 24 individual 64 Kbps channels.
In total they provide a speed of 1.544 Mbps (megabits per second) as noted above. Originally designed for PBX traffic, each channel can carry either voice traffic or data in DS-1 format. Your telco (telephone company) may permit you to buy just some of these individual channels, sharing the full T1 with one or more other companies. This is known as "fractional" T1 access, and usually permits you to scale up or back as business needs dictate. A special connecting device called a CSU/DSU and special 4-wire cabling is required for T1 access.
A T1 is overkill for most home users, akin to drinking right from the city water main. Fractional T1 is often used by medium sized businesses, and full T1by larger businesses and ISPs. An even larger pipe is the T3. Expect to pay anywhere from $200 to $500 per month USD just for the line, plus ISP charges.