Catnip is a member of the mint family. Although it is most associated with cats, it is also consumed by humans as a herb and as a natural insect repellent.
According to the American Humane Society, about 50 percent of cats have a gene that makes them susceptible to the effects of catnip. Scientists aren't exactly sure what physiologically happens to make cats respond to catnip, partially because cats respond differently based on whether they smell it or ingest it. It acts as a stimulant for most cats when smelled. When it is consumed, however, it acts more as a depressant.
The general belief is that catnip somehow facilitates a similar neurological effect to the natural pheromones that make cats happy. The effect, however, is short lived, lasting only about 10 minutes. It has no effect at all on those cats that are not genetically wired to respond to it or in kittens that are less than 3 months old.
Some people also find catnip calming when consumed, and it is sometimes used as a natural sedative for this reason. As an insect repellent, it is extremely powerful, but its effects are somewhat short lived in that capacity as well.