Mastering is the process of taking the final stereo mix and enhancing it further making it ready for replication, downloads and for commercial use. You will need to mix the tracks to the best of your ability and provide stereo files. Treatment tends to be broad and access to individual elements of the mix is limited if not impossible. It is always best to fix things at source rather than at the mastering stage.
Mixing is blending every individual element of the track (kick, snare, bass, guitar, piano, vocals etc.) to make a cohesive final mix. Every channel can be treated differently allowing for processes such as EQ, compression, reverb, automation etc.