A channel service unit (CSU) is a device that connects a terminal to a digital line. A data service unit (DSU) is a device that performs protective and diagnostic functions for a telecommunications line. Typically, the two devices are packaged as a single unit, CSU/DSU.
You can think of a CSU/DSU as a very high-powered and expensive modem. Such a device is required for both ends of a T-1 or T-3 connection; the units at both ends must be from the same manufacturer.