Mastering is the final step in the production of your recording, and in many ways the most important. It's how you make all the songs on an album play as a cohesive whole, although mastering touches can be applied to a single song as well.
Yes, one of the things we do in mastering is to decide how loud the track should play; that is, how loud the average level is compared to maximum possible level. If I had to pick one desert island plug-in, it would be this one: LOUDER, from the Waves OneKnob series. It's a marvelous boot-strap compressor that melds a mix together in a very satisfying way. Of course, I have a few other ones to choose from, too.
Mastering is such an important step in a commercial album that it's common practice to send all the final mixes to a mastering house, where a dedicated mastering engineer can optimize the sound of your album with fresh, objective ears. The top mastering engineers have forgotten more than I'll ever know about the art of album mastering, but the painstaking manner in which I record and mix lends itself to a relatively inexpensive mastering process and for the price you won't find anyone better.
If you wish to take that extra step and send your mixes off to a mastering house, I certainly have no problem giving you all the files and documentation necessary.
Here is a website where you can learn more about some of the best mastering engineers in the business: