Simply put: Photoshop CS is the newest version of Photoshop. Elements is the "lite" version of Photoshop.
If you own no version of Photoshop at all, then buy Elements because it will probably do everything you need and only costs $99. If I'm not mistaken, certain scanners come with Elements for free (not a bad deal). I accidently installed Elements when I bought my Epson 3200 scanner. All this did was take up hard drive space becuase I already had Photoshop.
If you have an older version of Photoshop then buy the Photoshop CS upgrade. It'll cost a little more but this will allow you to continue to buy upgrades.
By the way, you can call Adobe sales direct. I have always found them to be very informative and easy to contact (unlike other companies out there).
EDIT: wordmunger got there before me!!