I was an audio engineer all through the 90's, I've listened to a lot of reissue/remastered CD's.
Seafroggys is correct wit the difference between "remaster" and "remixing'.
Here's my take and experience, two things factor in the remaster:
The "original master" was used/found. As addressed above, virtually all 1st generation CDs were made from and eq'd and compressed copy or copy of a copy of the original master that was suited for vinyl, Also, the low end was often rolled off to reduce the groove spacing and allowing the lacquer master to be cut "hotter" and fit more time per side, as you can see vinyl is a very limited format!
Second reason: Advance in "A to D converters": this is the biggest leap in technology and audio quality (1st. generation converters were horrible): every new remaster goes through the latest A to D converters, as these have evolved, they have become more and more accurate, more transparent and most importantly, squeeze more and more resolution into a 16bit 44.khz. format, the difference is startling!
What is mastering?
Mastering is actually relatively simple, at its most basic, it's a transfer process to the final format used for duplication. But wait, there's more, in a perfect world that's all that would happen, a straight transfer, but no 2 track master is perfect, the mastering engineer goes over each song with a fine tooth comb, mainly adjusting levels and "tweaking" the track with eq and compression/limiting. Limiting is a really useful tool for digital mastering for CD/DVD, you can knock down an errant peak that happens only once or twice in the whole song and bring up the overall level 3 to 6db without really affecting the overall quality, this has a big impact to the perceived sound, especially for CD's where your limited to 16 bits of resolution.
Traditionally, this is not done to classical music, you leave the peaks in so the music is not altered in any way.
Ok, so why do "they" keep re-remastering albums??
Many reasons, sometimes the original master wasn't available but hey found it in someone's closet! More often it's new technology that triggers the label or band to remaster/re release an album (along with the revenue it generates), DSP (digital signal processing and editing) is so powerful and prevalent now that it can be a great aid in old recordings, very often it's used for noise reduction, removing clicks and errant noises, 60hz hum, and editing.
Finally,
In the late 90's early 00's, there was a new format that was next step in fidelity and was supposed to replace CD's, the DVD -A.
This was an audio only format (24 bit 96khz), The intention was with this level of resolution, you could capture the best analog recordings and preserve them. Analog multitrack tape machines can easily extend out to 40khz, I've set up some 2 track machines that you would mix down to can reach 50+ khz! This is easily an octave and a half above adult male hearing range.
So if you have a DVD player see if you can find a copy of your favorite album, they are out there, just getting very rare!
en.wikipedia.org