Equalization is a process used when mixing and mastering. The two are similar but also differ in how an engineer approaches using EQ in each situation. In mastering only the stereo mix is usually available. While in a mixing situation all of the single tracks will be able to have EQ applied individually.
With only the stereo mix available for adjustment in mastering, there will be a tradeoff with balancing making the required adjustments without adversely affecting something that is undesired. Enhancing one aspect of the mix might negatively affect another. The engineer has to be careful with how the equalization is applied for the mix as a whole.
If a vocal is say not bright enough in the mix then the engineer might add some tip end to the mix. This could help the vocal stand out nicely but at the same time create a situation where the rest of the mix ends up too bright. The key would be to choose frequencies that enhance the vocal but leaves the rest of the mix unchanged as possible.
Another trick would be to use M / S mode which can separate the mono and stereo parts of the program material. Now for instance the vocal which is panned center can be affected without changing the frequency balance of the stereo parts of the mix. This allows for more precise control and less tradeoffs.
The main goal in general with equalization in mastering is to arrive at a nice tonal balance that is both pleasing and accurate across multiple playback systems. The mastering engineer will test playback on more than one set of speakers to see if the master translates.
The music should also be clearly heard on a variety of sound systems from iPods to radios and night clubs. Every monitoring system will have its own biases to the frequencies portrayed. So it is up to the mastering engineer to create a flat enough balance so that on average the mix sounds correct on all of these systems, when applicable.
There is also the issue of knowing each genre and bringing out the best of the music within the style of that genre. Dance or hiphop music needs more top and bottom frequencies to give it the excitement required for it to be effective. A rock track might need more mid range to accent the guitars and vocals.
Two main types of equalization exist, parametric and shelving. Parametric employs three controls, frequency, gain, and bandwidth. This can be helpful for surgical situations that require very precise changes. The next type is shelving which generally control the high or low end of the spectrum. They are usually gentler and can affect the overall tonal balance of the material.
This has been a basic look at how EQ is used in audio mastering. Although somewhat similar to how EQ is used in the mixing process these are the main areas where it differs.