A decoder generally takes a binary number and tranlates it into a specific output. 3 pins are decoded into 1 of 8 pins going high. This is fundimental to a multiplexer, but only in that it is part of a multiplexer. A multiplexer takes a signal and uses the binary number to switch it to one of 8 pins. In the case of an switching analog multiplexer it is bidirectional, think in terms of a rotary switch.