Mastering in more Detail:

Focal Speaker PicMastering  is the last creative process in the music production chain. Mastering can add clarity, punch and body to a mix. Common tools used in mastering are: equalization, compression, expansion, image tools and limiting.
Most importantly though it is about listening to each track and applying the correct process needed for that particular track, not about applying the same default setting for all. 

The Mastering Engineer therefore may choose to use one or all of the above tools to get the track sounding as good as it can.
The mastering process also helps your music translate well to the outside world, i.e. it should allow your tracks to sound good on as many different sound systems as possible - ipods and desktop speakers, home hi-fi systems, car stereos, Radio / TV transmissions and club sound systems.

Mastering is also the last chance to repair any problems that occured in the recording process, ie click/glitch removal, de-essing and noise reduction etc.

It is also useful as it allows you to get an unbiased perspective on your tracks, as sometimes when mixing for days on end you are so 'close' to the mix that it becomes hard to hear the mix as a whole.

Another aspect of mastering is the actual compilation of the CD master. Not only do the tracks need to sound great individually, but the album needs to 'flow' nicely from one track to the next in terms of both overall tone and level.

A Small note on Loudness:
If you want your music very loud I can provide this, but bear in mind that by doing this you are limiting the dynamic range and punch of the tracks.
It also may become tiring to listen to after a while, and may produce nasty artifacts if later followed by more compression ie radio transmission or lossy encoding (mp3).

Learn more about The loudness War:
Ian Shepherd - Dynamic Range Day - The Loudness War
Wikipedia - The Loudness war