Mixing is like an all around creation of harmonic flow of a track aka leveling and effects processing (eq, reverb etc.)
(Beat mixing and full tracks with vocals are similar but different also. Like in full track mixing, you have to deal with how the humans voices, which are frequencies, interact with other instruments and sounds in the instrumental/instruments/sounds. With beat mixing your pretty much just level all the instruments, which are frequencies again, and maybe adjustments to make them sound whatever way you are trying to achieve or give it an interesting effect/sound.)
Mastering is like a step further in leveling, general overall audio repair/clean-up, song-to-song comparisons and getting it to a CD quality format.
Generally, you become a good, then great mixer, then choose to stay and advance with mixing or become a mastering engineer. Think of it as a mixing engineer and a mastering engineer use the same set of knives, but make didn't cuts in their own respective way.
Generally, again, if the mixer did his job to spec., the mastering eng. should only have to devote a small portion of his time "cleaning up" the audio/what the mixing eng. didn't do/catch. It can also involve advanced techniques like audio restoration, click/pop error removal, dithering, limiting, light overall eq'ing etc.
Mostly should be a/b or a/b/c comparisons (for quality and leveling purposes) and getting it down to the technical CD quality format 16-bit/44.1k.
That being said, focus on develop your mixing skills. 80% of the tools you use for mixing, you will use for mastering, but just in similar, but maybe different ways.
The simplest non-technical mixing advice I can give you on how to mix is: imagine the track is in a room and all the instruments are in the center of the room. Now you want to not only place the sounds in specific places so that they don't clash with other sounds OR so that they at least mesh with the other sounds in the same placement/area....and also you want to level the sounds after they are placed, Turn down (cut) some to let them play a background roll and/or turn up (boost) some to give them more prominence.
The other side of mixing is effects processing (delay, reverb, compression, limiting, etc.) All these will have to be learned separately, then you will understand how they all correlate to each other. They are just your different knives in the set and all serve different cutting purposes.
You should get to a point when you know what the sounds need. They may sound to screechy or high pitch, so you use you EQ to make a freq dip (cut). Maybe your drums have no punch/presence, maybe you'd use a compressor or limiter to bring them up. Your hi hats sound thin/uninteresting, maybe add some reverb to give them a signature sound.
They can all be used, or you may only have to use one effect. You are just trying to get it to sound a certain way or other times, fix a sound to sound.
I hope all that helped.