With TDMA, used in IS-136, and most GSM systems, each user occupies a radio frequency for a predetermined amount of time in an assigned time slot. Calls are combined or multiplexed into a digital stream by the base station. It assigns these chopped up bits and pieces into an efficient order by putting that information into the right time slots at the right time. Most TDMA based systems use two slots out of a possible six. Multiplexing combines several different calls into one coherent digital stream.
Channels handle the call processing, the actual mechanics of a call.
Don't confuse these data channels with radio channels. A pair of radio frequencies makes up a channel in cellular radio. One frequency to transmit and one to receive. In digital working, however, we call a channel a dedicated time slot within a data or bit stream. We'll go over this again soon.