Mixing is the process of taking the individual channels, or sounds in a record and adjusting each one on it’s own so that they fit together perfectly. Getting this step is right is paramount to achieving a professional sounding record.
Mastering is the process of adjusting the overall EQ, dynamic range and loudness of the master channel (all individual channels combined). In the context of an album mastering also serves the purpose of creating cohesion between all individual songs. The final step in the mastering process also involves choosing the right bit depth, sample rate, dithering and output format to prepare the record for media.