An encoder refers to a device that is used to change a signal or data into a code. Whereas a Multiplexer or mux is a device which performs multiplexing or it takes information from more than one channels and outputs into a single channel. Thus a multiplexer is basically a kind of encoder where its function is to combine multiple inputs into one output.
In Multiplexer both inputs and output are of same code type...In encoder the inputs is of one code type and outputs is of some other code type
Basically both encoder and multiplexer are from same category. There is a little difference that encoder convert the machine language to binary where as multiplexer convert the both coding.
Encoder is denoted by 8:3,4:2 where 8,4 are input and 3,2 are output