Slots hold individual call information within the frame, that is, the multiplexed pieces of each conversation as well as signaling and control data. 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. 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. A channel sends particular messages.