I'm certain someone else will give you a much better and more detailed answer, but the difference is mainly that a pressed CD is of 'professional' quality and will thus last longer than a CD-R, which is a low-priced consumer disc for recording data, images or music at home (and usually breaks down after a certain amount of playtime). To produce a pressed CD, you need professional hardware which is obviously more expensive than a common (high-priced) PC. So it's got nothing to do with the recorded material itself, but with the way it was burned onto a CD, and what kind of CD was used (professional or consumer type).
These terms are frequently used to discribe bootleg CD's - a pressed disc means a professional approach of burning (though evenly illegal if you do not own the rights to the material), while a CD-R could have been made by anyone who owns a PC, so chances are it is a cheap consumer's copy of a 'pressed' CD.
(If I made a mistake in my explanation, feel free to correct me.)

rinse_dream, January 18, 2006; 4:02 AM

The main difference is durability. I noticed that the CDR's I made with my computer for my
own use got unplayable after a year or so, especially when I attached a label sticker to them.
That's why I always buy original 'legally manufactured' CD's.

For my own compilations (and transfers from vinyl to CD) I bought a Philips Audio Compact
Disc Recorder. This makes copies from LP's or CD's in 'real time' without the use of a
computer. The discs are different from ordinary CDR's and have an 'Audio-logo' on them. I
presumed these copies are more durable than ordinary CDR's but I don't have it long enough
to check this. Also the CDR Audio discs are more expensive than the ordinary computer discs.
These CD Audio writers also have a protection built in against making copies from copies. You
usually can only make copies of original CD's. A workaround is sometimes to record a CD
'analog' instead of by optical or coax cable. If anyone has experience (good or bad) with the
durability of these Audio CDR's I would be interested to hear it.

Anyway, I don't trust CDR's since virtually all of the ones I have are now ruined (usually
some unbearable noise or cracking sound will accompany the music). And I hate mp3 tracks,
I would never buy them. If your computer crashes you lost your purchase and the sound is
compressed. I will buy genuine CD's as long as they're around (and it's anybody's guess how
long that will be).

chris, January 22, 2006; 1:19 PM