High poly models got a large amount of faces, like 10000 or something. There ment to be high detailed and mostly made for movies, pictures etc. Low poly models have a low amount of faces, ie. 100 or something. Those models are ment for background objects or for games where you have the keep the poly count low. When choosing for low poly model you should spent more time making/finding the right textures, so it's still looks decent. For example: you could model a high poly house with windows and doors etc. and then use a simple brick wall texture. Or you could make a low poly house, with four wall and a roof and spent more time in the texture. The texture should give the details to the house then.
There is a everlasting stuggle between those two. A good modeler should have to find the right compromise between high poly models and high detailed textures.
