Well most commonly they have to do with the number of plys in the tire.
P is a normal passenger tire.
LT is a "light truck" tire.
Within LT there are also different ratings; C, D and E most commonly. I personally have some LT255/75/17 C rated BFG MT's and love them.
Simply put, the higher the rating, LT v. P/ E v. C, the rougher the ride, louder the tire, higher the required air pressure. On the flip side, the higher the rating, the stronger your tire, esp in your sidewalls where strength is really important.
Hope that helped!