Below are samples of before and after mastering done by 4daCulture. The file sizes were rendered to 320kbs 44.1khz mp3 files which is far better than itunes standards. This should give you a good idea of what mastering is and how it affects the overall song quality. The original sample volumes were leveled to match that of the mastered sample. Take a listen with a quality pair of headphones or hifi stereo system to really hear the differences.