Catnip mimics feline sex hormones, so cats enjoying this substance will often display behaviours similar to a female cat in heat (although both male and female cats can experience the effects of catnip).
These behaviours can include overt signs of affection, relaxation, and happiness. Other cats will display active behaviors, such as playfulness or sometimes even aggression.
Cats eat it, roll in it, rub their face on it, or sleep on it. It makes many cats playful and skittish. Rolling on the plant, or crushing it by chewing, releases Nepetalactone which cats find stimulating.