Catnip, or Nepeta cataria from the genus Nepeta, is a species of plant that's commonly used in cooking and brewing herbal teas. Oh, and about 66% of cats go crazy for it.
An essential oil called nepetalactone that's found in the stems and leaves of the Nepeta plant can have powerful effects on kitties (and not just domesticated cats, but other species such as tigers, panthers, and leopards). Sniffing the plant is thought to stimulate the receptors in a cat's brain that respond to "happy" or pleasurable pheromones, while eating it can bring about mellow effects.
Fun fact: catnip is related to the cannabis plant; in fact, author Michael Pollan wonders if both cannabis and Nepeta cataria both use their respective chemicals to "confuse" their pests as part of an evolutionary strategy:
"Killing pests can be counterproductive, because they breed or select for resistance very quickly. This happens with a lot of poisonous types of plants, as it does with pesticides. But if the plant merely confuses the pests or disables their memory, it can defend itself against them overindulging. Pure speculation, as I say in the book. It occurred to me that it might help explain what's happening with cannabis, which of course also disables memory."