I am going to post a comment I found on reddit I found interesting. Not sure how good it is.

I'm a professional mastering engineer and I get asked this question at least 2-3 times a month. I'll copy and paste a response I wrote here:
Mastering nowadays is thought of as making the song as loud as possible, but this isn't really all it is. While increasing volume is a component of mastering, it's really more about finalizing the song in a way that maximizes fidelity, clarity, and translation to other devices and mediums. This all has to start in the mix - you can't use mastering as a panacea to make your song sound good on both ear buds and club systems.
Anyway, out of the introductory stuff and into some detail: a rudimentary master chain might look something like this:
EQ > compressor > EQ > multiband compressor > filter > limiter > soft clipper.
Every mix needs a different master, which is why people look down upon "preset mastering" for $10-30 on the internet, but this signal path should give you an idea of what mastering does.
The first EQ could be used to notch out some problematic frequencies - maybe a small range that has some wild resonant peaks that will negatively impact the compression.
The first compressor could be used to start delicately taming some transients - maybe a dB of gain reduction every now and then.
The second EQ could be used to even out the spectrum a bit - maybe the mix is lacking in the 750-1200 Hz range by a few dB due to a speaker coloration, so you could boost a small amount there.
The multiband compressor could be used to treat a small section of the mix that's too harsh. For example, maybe your cymbals are just a tad too energetic sometimes. Dial in on them and compress. Keep in mind that arbitrarily using a mband will ruin your mix faster than you can say Sausage Fattener. Always know why you're using the multiband compressor, and if you're mixing too, a lot of issues can be solved there and you may not even need it.
The filter would HP at 30ish Hz, and maybe LP around 18khz. Straightforward.
The limiter would ensure you don't clip, and give you a bit more volume. Don't cram your signal into the limiter - let the mix breathe. A dB or two of flickering gain reduction at most. Use a couple limiters with different attack/release to get some more volume. Only a dB or two on each.
Lastly, the soft clipper will allow you to get a bit more volume if you want it. They often sound more pleasant than a limiter.
This is a very basic rundown of a simple master chain, designed only to illustrate what mastering does and how it differs from mixing. You will need to experiment and mess around with values and signal paths to fit your music and mixes. Mastering is a very large topic and it cannot be fully explained in a Reddit post.
Feel free to ask if you have any questions.

Credit goes to http://www.reddit.com/user/_Appello_