Decoder – (Might also be known as Set-top Box, Converter, Receiver, Descrambler, Gateway Decoder Platform) - It’s a device and/or program that changes encoded data back into its original format, or the format necessary. The term is often used in reference to MPEG video and sound data, which must be decoded before it's viewable. Most DVD players, for example, include a decoder card which sole function is to decode MPEG data. 

Encoded data typically needs a "decoder" at the customer's receiving end to change it into a signal, or data stream that a TV or other applicable device can understand.  (Analog TVs don't understand digital signals unless the signal is "decoded" by a computerized device for them.)


A set-top box typically contains decoding technology. A digital set-top box and its integrated television
counterpart, might have these 3 decoders:
 

Video Decoder - It decodes the video packets into images that can be viewed.
Audio Decoder - It decompresses the audio bit-stream. 
Data Decoder - It helps you choose between the potentially hundreds of channels and Interactive TV services.

Also see http://en.wikipedia.org/wiki/Decoder and Set-top Boxes.


Codec

Encoder

Lossy Compression

Lossless Compression (Nonlossy Compression)

Multiplex

See the Interactive Television Institute.