BRDTS is dead on. Thanks for the great explanation for the uninitiated!
That said, I'd like to chime in and give you this bit of opinion (which you may have heard elsewhere): the only way to enjoy the Beatles (Please Please Me through The White Album) is in mono. Those were the mixes that the Beatles were present for, and given great care to from the engineers and Martin. If you look at the Beatles sessions book, you'll realize that the engineers spent very little time on any stereo mix, if they made one at all. If you haven't heard the original UK mono mixes, you're in for a treat: Helter Skelter rocks harder than you'd think the Beatles could rock, the flange on Lucy in the Sky With Diamonds is far more intense than the tepid stereo mix, and in general, the band is more "together" and the overall sound is punchier.
Lots of gray market CDs are available, taken from mint original UK LPs or Japanese LPs. You can find some of these on Usenet, especially in the Beatles binaries group. Look for releases by Doctor Ebbetts, Mirror Spock, and my favorite, the Millenium Remasters.