Mastering is basically about 3 elements.  Those elements are equalization, dynamics control, and loudness management.  You could add a fourth element which is effects, but overall the first 3 elements are involved every time a song is mastered.

An equalizer or EQ is used to turn down the frequencies that are negatively affecting the music or song, and to accentuate the frequencies that are positively affecting the music or song.  Also, an EQ is used to balance the frequency spectrum of a song or music.  In other words, you don't want the high or low frequencies of a song to be too loud or present, and you don't want the middle frequencies to be too loud or present.  You want them all to work together while being balanced with one another.  This gives the music the best chance of sounding good on any medium, or on any playback device.

Music is very dynamic.  This means that the sound moves around, and it moves around a lot.  Just think of a human voice.  When communicating, the human voice goes up and down.  It get's higher pitched at some points, and lower pitched at others.  At times a voice gets louder, and it gets more quiet at others.  The same thing happens with a song or music, and those changes need to be controlled to some degree to avoid sonic chaos.  During mastering, the dynamics are primarily controlled with compression.  You want the sound to move, just not too much.  A compressor gently controls the dynamics of a song without crushing the life out of it.  It also acts as another way to balance out the frequency spectrum of a song or music.  The balance is achieved by balancing the overall tone of the song, as opposed to specific frequencies like with an EQ.

We want our music to be loud, but not too loud.  This is what loudness management is all about, and this is achieved using a limiter.  When it comes to controlling dynamics, a compressor is Dr. Jekyll, and a limiter is Mr. Hyde.  The compressor is the easy going dynamics processor, while the limiter is the strict, no nonsense; my way or the highway type of dynamics processor.  Some peaks may get by the compressor, but they wont get by the limiter.  This is exactly why at Flamebridge Music, we don't over use the limiter.  Over use of a limiter is a good way to negatively affect your music, and that is the opposite of what we want.  Compression primarily handles the body of the sound, and limiting primarily handles the peaks.  Working together, compression and limiting makes the quieter elements of the song louder, and the louder elements of the song quieter to balance everything out.

As for the effects, most of the time that involves stereo imaging.  Stereo imaging is used to make a song or music sound bigger, and more wide spread.  going the other way, if the song already sounds too big and too wide spread, then stereo imaging does the opposite.  And if the song already has a stereo image that is just right, then no stereo imaging is applied.  At the end of the day, it's all about what works best, and what sounds best.

All of these processes are included when Flamebridge Music masters you songs or music.