Encoder, Mux, Decoder and Demux

MULTIPLEXER: The device which chooses the input channels. It has several
input channels but the output signal from all these channels is sent to one
single output channel.

DE-MULTIPLEXER: The device which is chooses the output channels. It has a
single input channel but outputs the signal from this single channel to several
output channels (one channel at a time)

Decoders are essentially DeMUXes. However, encoders are not essentially MUXes.
Roughly speaking, decoders convert from binary to unary, while encoders convert
from unary to binary.

Encoding:

The way in which the computer data is represented is known as encoding, there
are several encoding techniques (Unipolar, Bipolar and Biphase).

Creating a video file in a special format (e.g. DivX, MPEG-2, MPEG-4, ...) -
Youll need a DivX Encoder in order to be able to create DivX files and a MPEG-2
Encoder in order to be able to create MPEG-2 videos.

Opening/playing/reading/watching a file (Decoding) and making a file (Encoding)