The human nose can distinguish more than 10,000 different smells.

There are hundreds of different kinds of neurons in the nose that specialize in recognizing different odors and transmitting those signals to the brain. This occurs when molecules of the things that you can smell travel to your nose. Whatever thing you are smelling (a flower, a piece of fruit, an onion, etc.) gives off light, volatile chemicals called odorants that make their way to your nose as they evaporate. These chemicals trigger the neurons in your nose that allow you to recognize the smell. Specialized smell neurons exist to pick up on different kinds of smells, allowing us to distinguish between over 10,000 different odorants.

 
Source: How Stuff Works