Depends who does it.
Remastering means going back to the original master tape and creating a new master source... Hold on... let's *rewind* a little (or maybe someone else can describe it more clearly/accurately!)...
The tape that contains the final mix by the engineer is the master tape, but the master source could be the "Father" platter used in creating a vinyl record, or the source CD that the retail copies are taken from.
Mastering is essentially the process of equalising the sound so that it still conveys the music properly on the new medium; Vinyl has a smaller dynamic range than tape, so traditionally, engineers would roll off the treble and bass (especially the bass, which had the tendency to make the grooves run into each other if it was too high - Chris Squire must've given a couple of engineers a headache!).
For CD, it's still a process of EQ'ing - but as I understand it, the process is a lot more complex and depends greatly on who is doing it and for whom;
For the iPod market, the music has to have a fairly flat dynamic range - people don't like to constantly turn the music up in quiet bits or down in the loud sections.
For the audiophile, this simply won't do - every aspect of the dynamic is relished.
Hence you see "Audiophile pressings" and suchlike - almost invariably of a much higher quality, and mastered at half speed so that more data is transferred from the source.
In summary, it depends. Re-mastered can simply mean re-eq'd from the old analogue mix to suit digital media - but which digital media is anyone's guess.
Some are good (e.g. the 24-bit reMastering of Marillion's "Script for a Jester's Tear"), while some are awful (e.g. the Quiex pressing of Led Zeppelin II, which is faster, and the pitch is not preserved, giving an awful quasi Micky-Mouse sound to Plant's vocals and suchlike).
So it depends