Decoders in Digial Electronics

Read this from Wiki and then try to be more specific about your queston or go to chapter three of your textbook.

A decoder is a hardware version of the mathematical concept of mapping. It takes one type of information and decodes it into another type of information. Usually this is a unique mapping scheme. In other words, one and only one unique input produces one and only one output. However, this does not have to be the case. A simple example of a decoder that accepts multiple information forms to produce the same output is the easy translation of your misspelling of the word "Digital" by accidentally typing "Digial". Our brains (and the spell checker) decode "Digial" to mean "Digital" along with the word "Digital".