"Remixing" vs "remastering" recordings

Remixing:

http://en.wikipedia.org/wiki/Remixing

Remastering:

http://en.wikipedia.org/wiki/Remaster

They are two very different processes. A remix is more involved in that it also includes a new mastering session. A remix is only possible for recordings where the original recording sessions were multitracked. The remix engineer would have to go back to the tapes (presumably) containing individual instrumental or vocal tracks. This allows the engineer to alter the level, EQ, compression, reverb levels or effect processes for individual instruments. New instrumental tracks can even be added. It is then possible for the engineer to completely reimaging the album. Once the new mix is finalized the next step is the mastering process. Typically remixes involve the artist and a producer.

Good explaination by Onhwy61. Also remixes are done for DJs to be played in clubs. It's different than the original tune on the album. For a club mix, the Producer/engineer will make a "hotter" mix and may add drum machine, samples and other effects. Also the track will most likely be a longer running time.