Low density ram chips work with a wider range of motherboards, high densitiy have more chips but work with less.
If you put a high density stick into a board that runs only low density it will show the ram at half of its actual size. (if i remember right its because the ram chips are doubled, could be wrong though)
High density has more chips on it if that helps. For actually identifying what the particular ram is there are lots of hardware ID programs on the net. I really havent used any so good luck there