These are just my understandings. Please correct me if I'm wrong anywhere!
IIRC, there are two types of LED back lit displays: the more expensive LED matrix, and the less expensive LED border-type back lighting. CCFL back lighting is also border type. (Not sure what the industry term for border back lighting is)
Border back lighting for LED and CCFL is probably similar. Many of these do adaptive brightness adjusting which helps make dark scenes darker, but sadly it does not help the contrast (ie a half-white half-black screen will still look awful).
LED matrix, I believe, is set up to dim the back light for specific portions of the screen so one area can be dark and another area can be bright. This can look way prettier, but requires a thicker display and is more expensive.
Does this all sound accurate to those more knowledgeable?
Edit: I did a little reading, and apparently there are two types of matrix, too. There is an "RGB matrix" which does local dimming, and a "full array" matrix which does not. Not sure if these terms are universal, though, so it might be hard to determine what you are buying.http://en.wikipedia.org/wiki/LED-backlit_LCD_television