Theres a lot of info here:
http://en.wikipedia.org/wiki/Replaygain
But basically, in album gain mode every track in an album is set to the same volume. In track gain mode, every track gets the same volume (usually only used with mixes or shuffle since this will cause really wierd effects on gapless albums). +12dB would try to make every album 12dB louder, but you can’t actually go nearly that loud so either it would saturate (if clipping detection is enabled) or massively distort (no detection). I don’t recommend going above a few dB if you want replaygain to actually normalize volume.
