What exactly is Mastering?

When I first started producing music, a lot of friends said that my older tunes needed this.

What exactly is it?
How is it done and
What makes it so important?

I've only been learning how to produce for a few months, but from what I've picked up during that time, mastering seems to refer to taking the mix from its "raw" form and adding all the final touches that improve its sound quality. Mastering is always done last when making a song (though I often cheat by running basic tests to see how well I can master a track when I only have a beat and bassline going, just to give some idea of how it will sound in the end.)

I've been reading a lot, and there seem to be some common rules you have to follow. Most of my following commentary, though, is just my experience by just learning it myself and after getting some ideas from what I've read. Often, it's a VERY bad idea to compress the whole mix. First, what's the point? If you want to raise the loudness, don't use compression, use peak limiting, as this removes clicks and can raise the overall volume of the track by bringing the loudest sounds closer to the softest sounds. Too much peak limiting can severely damage the sound waves by clipping too much and it will sound very harsh. So, definitely play around there first.

Another angle to mastering, as far as I know, is to give the EQ more definition. This changes the EQ for the WHOLE mix, mind you, so if you're trying to make a single instrument stand out, you're better of just EQing that single track. But EQing the whole mix can be nice for getting the whole mix to sound richer and cleaner.

Yeah, but don't cling to what I say. I'm not exactly pumping out hits, so my knowledge is limited on this.   

It's just taking different elements of the track such as the bassline, lead, percs, ect and EQing and managing the levels on each one. It's important because if not done correctly you'll have parts of the song that clip or are too quiet making it a terrible production regardless of how well its been put together. But don't get too caught up in mastering. There are some people out there that think its the silver bullet to making a great track, when it's really just another step.

Here is a quote to live by:

You can work very hard at dressing up a boring, heartless, and forgettable composition with nifty sounds and effects, and EQing and compressing it so that it sounds great on a club system. It will probably even get people dancing as long as it booms out of the speakers with plenty of force. But in the end it will still be boring and heartless.

when your tunes arent professionally mastered they sound like crap especially when played over speakers, of mixed in with other tunes.

Mastering does a couple basic things (I am a professional audio engineer so I do know!). I'll go through a basic mastering processing chain. A mastering session starts of with all of your stereo mixes that you plan on mastering on separate tracks. You master all your tracks together as to get a cohesive sounding album or EP or whatever.

1. Multiband EQ. This basically allows you to EQ the overall frequency curve of your tracks. An EQ curve with lower frequencies brought up will bring out instruments like a kick drum, bass, and other lower freq. isntruments. A high frequency curve will bring out hi hats, snare hits, and other high end things. This first part of the chain can really shape how your mix will sound.

2. Multiband Compression - Compresses the levels of individual frequency bands. This also allows you to shape the overall sound by accentuating certain frequency spectrums in a song. For dance music, you wanna bring out the deep bass (50-250 Hz) and high mid range (2000-5000 Hz) and maybe the high end too. Probably the most important part in making multiple tracks sound like a cohesive work.

3. Limiting - Limiting is basically like an extreme overall compression, and helps bring a mix up to standard CD volume levels. Depending on the limiter you use, it can add a certain characteristic to your music. Limiting is used heavily these days. If you listen to a trance song from 1996  and a trance song from 2009 it is just so much louder. Limiting is really important if you want professional sounding mixes.

There's more but those are the basic 3 elements. Hope that helps!

The thing about limiting that can really rub me the wrong way is that it feels that many releases now limit to beyond 0 dB, so you have terrible clipping as a result. True, it will be louder, and really even that I don't think is a set-in-stone "benefit." It's more of a commercial decision. But then again, I feel that that is why we have volume controls to dictate how loud the listeners will want them. But there's a price to limiting, which is that it is taking away the dynamics and a sound you call your own. And it has really gotten to the point where I can't even bring myself to buy certain tracks. Check out the preview for Sindre Eide's Essentia on iTunes, and there is horrible crackling and booming during one of the most emotional breaks during the song! That's even on a flat equalizer. Not that I really liked the track (one can check my comments elsewhere on this track), but I sat there thinking, "You know, I would refuse to buy this." It's an absolute earache on high volume. Same with Helsinki Scorchin', which has boomyness, and a few others. Were it not a stellar piece of music, I wouldn't get a track. I also refuse to ever send mp3 demos to labels who I'm sure would force me to make my track sound abrasive and as though it went through a cheese grater. Though I don't know many who don't limit anymore, so I'd revise that to say I would only send demos to those who don't limit as much. It's a good thing programs like Audacity are around so I check check the dynamics of a track. As someone said once, dynamics are art, volume is business. I know, I'm cynical, but it is what it is.

Agreed...limiting is heavily abused, not only in electronic music, but in rock (see Metallica's Death Magnetic) and Hip Hop/Top 40 Pop (see everything).

Clipping and digital distortion are unacceptable always. There's never a mix that I do at the studio I work at where I think "You know, this track could use some audible digital distortion." That being said, limiters, when used right and not unabashedly abused, can make a mix feel so much more alive, and really take your mix to the next level.

No doubt, it's kind of getting out of control, and I predict a backlash in the whole "loud is better" mentality in the near future, but multiband limiters aren't going anywhere, and if anyone thinks their mixes have a shot without having a proper mastering chain on their tracks is seriously fooling themselves. It's a 1/4th of the recording process (after recording, editing, and mixing) and is just as important as anything else when producing.

That's why good mastering engineers get paid the big bucks! 

Thanks for that clarification on mastering. I, like DJRanz didn't know what it really ment to the business and how i hear thinks

Very interesting thread. Thanks! 
~L