Audio mastering is the final stage in the audio production chain and the procedure originated at the advent of the vinyl medium. After music was mixed (or simply recorded live straight to tape in the early days of recording) it would need to be prepared by a skilled audio engineer for the final distribution medium. In the case of a vinyl release (12 "LP's, 12" Singles, 7 "singles and EP's) the engineer had to adhere to strict audio technical specifications in order for the music to" cut "to a lacquer which would eventually become a stamper for the vinyl copies.
The vinyl medium has a number of very stringent technical limitations which require a very skilled engineer to ensure that the lathe that produces the lacquer is set up in a way that will not create technical problems. (ie record skips and head burn outs). In addition to these requirements the lathe engineer would sometimes be asked to try and make a cut that will reproduce loudly as to be competitive against other record labels tracks when broadcast on radio stations.
Obviously times have moved on and now our music is distributed in different formats such as CD's and digitally online as MP3's. One important aspect of mastering is to ensure that the music is as sonically balanced as possible so that the lows mids and highs sound right when played on any of the many different types of music systems they may be played on. One of the latest trends is for very loud sounding masters, with the development of "digital limiting" music could be pushed ever louder without too many negative audio side effects. However these days the technology has reached it's limits and as music is pashed ever louder for reasons of competition the music can end up having an increase of distortion, it's a hot topic amongst music aficianados and engineers alike. This has been termed the "loudness war". Mastering is still a very important procedure as much for quality control on a musical product as anything and there are many such mastering services offered on the internet. It is with some care that online mastering should be chosen, with serious consideration for the engineers experience, existing client base and equipment.