Catnip is a perennial herb that contains an essenstial oil that has a profound impact on the behaviour of domestice and other cat species.  It is estimated that approx two thirds of cats will respond to catnip.   A cat's usual reactions to catnip include sniffing, eating, licking, and shaking of the head. The next common action is to brush their chin, cheek, and body against the catnip source.  
Cats may also display hyperactivity, aggression, stretching, drooling, jumping, and excessive licking in addition to these behaviors. The "high" that is supposedly brought on by catnip usually lasts five to ten minutes.
Catnip is therefore a great training aid if you want your cat to be happy in a certain area of the house or want them to use a scratching post instead of the couch try popping some cat nip there or use a catnip spray to entice them to the area and feel happy.