The sense of smell plays a considerable role in the social behavior of a cat. That's why when two cats first meet each other, the first thing they do is sniff each other's noses. While hunting, cats don't rely much on their sense of smell, although with its more than 200 million olfactory cells it is far more developed than in humans.
A cat marks with her scent glands on both sides of the head near the chin, on lips and tail. Scent marks are left on trees, objects, other pets and even humans. We cannot smell this substance. Cats, however, take it as a sign of friendship. Intact tom-cats also leave scent marks with their anal glands which, unfortunately, is a lot easier for humans to pick up, even with our own inferior nostrils.