Mixing
Audio mixing is the process by which multiple sounds are combined into one or more channels. In the process, a source's volume level, frequency content, dynamics, and panoramic position are manipulated and or enhanced. This practical, aesthetic, or otherwise creative treatment is done in order to produce a finished version that is appealing to listeners.
Mastering
Mastering is a process that brings a mix up to a commercial level of loudness (taking into consideration the track's genre) and will generally include corrective equalization and dynamic compression in order to optimize sound translation on all playback systems.