Mixing and mastering are two parts of the professional recording process that confuse a lot of people. If you are recording an album that you plan to sell, a good mixing and mastering job is a must. If you are only recording a demo, you may be able to get away without mastering.

Mixing and Mastering: How It's Done

Now that we know mixing and mastering are the two base components of professional record producing, it's important to understand what each one actually is.
Mixing—Bringing Multiple Layers Together
The first term, mixing, refers to the process of putting multiple layers of audio together to make one final track, or to musically modify an existing track.
Mixing is basically tinkering with everything you have recorded to complete your songs. You'll do things like drop in effects, adjust fader, EQ your tracks and so on. Think of mixing as putting the puzzle together. You're putting together the parts of what you have recorded, making sure everything hangs together right, and putting some finishing touches on things.

Mastering—Optimizing the Overall Sound
Mastering is adding sparkle and shine to your music. The term refers to the process of optimizing each final track by compressing, equalizing, making stereo enhancements, or adjusting the reverberation (echo) effect. In a very basic sense, when you master your album, you're making sure that song one doesn't blow out the speakers while song two is barely audible. In other words, you want the levels of the songs to be similar and you want a general sense of cohesiveness to your recording.
Does that explanation of mastering sound a little vague? That's because it is. Apart from correcting obvious differences in volume for each song, mastering is an incredibly subjective process. In some ways, when it comes to mastering, some people believe that you either have the golden touch or you don't. For this reason, although there are programs that will help you master your recording yourself, paying to have it done professionally is a good investment if you plan on releasing your recording to the public.

When You Can Do Just One

If you're planning on using your recording for a demo, mastering is not an absolute must as it requires intensive knowledge and experience, which is costly when done by a professional. Mixing, on the other hand, is something you should make an effort to get done, no matter what stage of release your song or album may be in. You don't have to have a professional mix, but you should try to give your songs at least a rough mix when possible. Unlike mastering, you can do mixing at home. It requires practice and time, but with some dedication, you can get the job done.
If you want a more in depth look at record mixing and record mastering, plus advice on how to do it yourself, please visit About.com's Home Recording site.

Sourse: http://musicians.about.com/od/musiciansfaq/f/mixingormaster.htm