The difference between facilitated diffusion and active transport is that facilitated distribution occurs down a concentration gradient and active transport occurs against a concentration gradient. I understand that, as a result of these differences, active transport uses ATP so that it can bind to the carrier protein, however I don't know what is meant by going against a concentration gradient or down a concentration gradient.
In short, I would like to know the difference between going against and down a concentration gradient. A definition of what a concentration gradient is might clarify things a little further as well, since the definition I have doesn't help much.