Mastering
One of the least-understood parts of the recording process is the mastering process. It's certainly something that's easier to show than it is to describe. This section will (eventually) have a pre-mastered sample next to a post-mastered sample so that you can hear the difference.
Mastering people are usually not mixdown folks or recording engineers. They are a separate breed of people. The best description is that they have Vulcan ears, and can hear tiny discrepancies in sounds, and other aspects of the final recording, and can smooth things over and tweak the final sound. The mastering process is kind of like sanding and varnishing a carpentry project. Without it, the final project sounds rough.
Mastering takes the result of the mixdown process for each of your songs and does the following to it:
Normalizes the volume
Mastering will set the final volume of all of the songs so that they are at the same level. If this is not done, there could be jarring changes in volume between tracks. Most rock CDs are mastered to boost the volume of most tracks.
High quality EQ
The mastering process performs overall EQ on the finished tracks. During mixdown, you probably performed EQ on each track separately and you may have done some EQ to the master track. Mastering folks go beyond this and usually have more sophisticated EQ units. This can make the final product sound much "clearer."
Sets the order of the final CD
After working on each track, the mastering engineer will order the tracks on the final disc. They will also set fades in between songs and the better ones will give you a final disc that has special information for the replicators on it.
Other effects
There are a host of other effects that a mastering person will do to a recording. One example is a process that smoothes out the rough edges of a digital recording by putting the output through an actual analog device based on vacuum tubes. Sometimes, adding "crowd noise" to live albums is done in the mastering studio as this is sometimes a question of bridging between tracks that may not have been played next to each other live.
Mastering is charged on an hourly basis, somtimes at $100 or more an hour. So you will want to be prepared with everything that you need in order to save time. Here's what to do in order to make the most of the time:
Make sure that the mixdown is really done
Listen to your mixdown over and over again at different sessions and in different environments. Play it for different people, especially other musicians, for comments. A mastering person usually can't fix a track that's mixed too loudly or other items that can only be done in mixdown. Although they do have a surprising bag of tricks at times, do not depend on them to fix any mixdown mistakes such as a vocal set too low.
Bring the mixdown to the mastering studio using the right formats
Check the mastering house for the formats that they require. Make sure you bring them what they can use, whether that is WAV files, tape, CD, or something else. It's very expensive to schedule a time and then to have the music arrive there in the wrong format. (Note: The right answer is never MP3 files. MP3 is a low-quality version of your recording. A mastering house can't do anything with an MP3 file.)
Bring a document with the track order
Decide on the track order before going to the mastering house and bring some paper with what the order should be. Deciding the order while you're there can be expensive.
Don't clip the before and after sounds on a track
Some mastering houses like the before and after junk on a track, because that gives them something to analyze before the track starts. They will clip the tracks to the exact length and get rid of the junk.
Other than these, enjoy the process. Mastering is actually a lot of fun!