A PRI line is an end-to-end digital circuit. Sometimes also called Megalink, digital, or fiber, it is a TDM, ISDN type (circuit-switched) telecom service that is rapidly replacing the older style analog line. A standard PRI service in North America has 23B channels and 1 D channel - if added together you have 24 channels.
These 24 channels are comprised of 24 channels of 64 kbit / second, or 1.544 Mbit / second (24 X 64). The PRI is a similar service to the T1 - a T1 is made up of 24B channels, or 1.544 MB as well. The T1 has 24B channels, and the PRI has 23B + 1 D channel. The PRI is used for voice, and the T1 is used for data.