Mastering is the generalized term for what is technically "premastering" in the CD and Vinyl world.  Often referred to simply as mastering, this stage most commonly refers to the process of taking an audio mix and preparing it for distribution.
This procedure has numerous facets: balancing or shaping the sound of a record, maintaining consistency across an album, maintaining consistency in various playback scenarios and preparing the album for manufacturing and distribution.

Balancing and shaping the sound:

The goal of this stage is to remedy mix balance issues and enhance particular sonic characteristics, taking a finished mix (usually a stereo file) and doing any final stage tweaking required to polish the mix  This can involve attenuating or boosting levels and general “sweetening” of the mix. This process can require equalization, compression, limiting as well as adjustment of sidebands and the stereo image.

Consistency:

The flow from one track to another is important. Verifying that tracks are appropriately leveled guarantees that the listener's volume control does not need to be adjusted from track to track. Furthermore, having your album be of an appropriate overall "loudness" guarantees a consistency when the album is played back to back or on shuffle with other professionally released albums. While the "loudness wars" have done a lot of damage to the musical fidelity and dynamic range of many popular genres, the reality is many bands/artists want albums that are competitive or at least carrying enough loudness-density to make the album comparable to most other modern releases. Exceptions to this exist and there are many recordings that could benefit from the K-Metering (Katz) system for final loudness output. Konrad Mastering can tailor the loudness level to your vision.

Preparation for distribution:

This final stage insures the audio has been prepared for the destination target which can include duplication, replication, digital downloads or any combination of the above. This stage varies depending on the format required. In the case of a CD, it can mean converting to 16 bit/44.1 kHz audio through resampling and/or dithering, and setting track indexes, track gaps, PQ codes, as well as preparation of a DDP (Disc Description Protocol). For digital distribution the levels may need to be adjusted to prepare for conversion to AAC, MP3 or 24-bit hi-res files and include the required metadata.