Several oversimplified answers here, but DebC, a thorough pro, gave you the best one. Mastering is part science, part art. The science is what the others have alluded to. It is making sure the sound levels aren't all over the board. Mixing and mastering, as was pointed out, is where levels withing the recording are set. This is so there is the proper balance between the instruments and voices, etc. Mastering then would apply some EQ to bring the master levels into sync. The art comes in at this point. I payed a fair amount of money to have our first CD mastered and was pretty disappointed. They overcompressed everything. I made my mind up that I would never send something to be mastered again. On the second CD, I sat at the console with Paul Mills as he mastered. He is as good as it gets. We had some dialogue, but I would trust him completely with or without my being present. What is the takeaway? If you are going to have it mastered, and DebC gave you the criteria as to whether you should, make sure it is someone who shares and understands your sensibilities. Sandy Paton (legendary owner and recording engineer for Folk Legacy) hated mastering, he felt like it stole the soul of folk music acoustic instruments with compression. Yet if you listen to Paul Mills work, you know that, in the right hands, it is a wonderful tool.