In it's simplest form, mastering is simply the creation of a high-spec production master , however, more often it involves a subtle but dramatic creative process, requiring great empathy and skill on the part of the engineer. Every track is assessed on it's own merits and treated accordingly, but at the same time are brought together to work as a "whole", to make an album a consistent listening experience.
http://www.soundrecordingtechnology.com/services/mastering.h...

Playback features let you maintain your music library in one convenient place. The new AutoGain feature lets you automatically correct changes in volume levels on songs recorded from different CDs for a consistent listening experience.
http://mall-ca.realint.com/Software/Video-Music/7/

I personally don't think that the more recently remastered Donna Summer material (Bad Girls Deluxe Edition and Gold) sound as good as the same material on the original cd issues, so even if these albums have been remastered for this set it isn't much of a selling point for me. Not that the recent remasters sound bad, but they are a tad compressed and sound more futzed with -- ie, on Gold the tracks from various producers have more of a sameness to them than they do on previous compilations, causing me to suspect that the various sources were EQ'd to provide a more consistent listening experience.
http://www.stevehoffman.tv/forums/printthread.php?t=61905