Receiver - radio tuner/amplifier, usually these days with some surround sound decoding and processing built-in.
Decoder - something that takes an audio format, for example, and turns it into a form another product can work with. So the decoder in a Blu-ray player can 'unzip' HD audio soundtracks, and output them either as analogue audio or Linear PCM to an amp or receiver without the decoding built-in.
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