catnip is a plant, it's edible for kitties and their people too. you can even make catnip tea.
here's some info from Yahoo
What's in catnip that drives cats crazy?
Catnip can turn a usually reserved and haughty feline into a playful, drooling kitten after just a few sniffs. It's primarily the scent of catnip (not the taste) that cats react to.
The pleasure factor in catnip is the chemical compound nepetalactone, which seems to have a mildly hallucinogenic effect on cats. Nepetalactone is similar to a chemical found in the urine of female cats, and this may be why unneutered males have stronger reactions to catnip than females and neutered males. Some speculate that nepetalactone stimulates the region of a cat's brain associated with sexual behavior.
It's not just domestic house cats that have an affinity for catnip -- even lions, pumas, and leopards are said to enjoy a roll in the 'nip. But not every cat will get a catnip buzz. Kittens don't react to catnip until they are six to eight months old. And roughly 20% to 50% of all cats have no reaction to the stuff. Catnip sensitivity is genetic and may also be influenced by learning and experience.
For those hep cats that do get a kick out of the stuff, the good news is that catnip is harmless and non-addictive. So thankfully no Narcotics Anonymous meetings for Fluffy.