This article is from the Miscellaneous Cat Information FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Catnip is a plant that causes various reactions in cats. Very young cats and kittens will not be affected by catnip. About 20% of cats are never affected by catnip. It is not known why or how catnip has the effect it does on the rest of the cat population. It is a non-addictive "recreational drug" for cats with no known harm to the cat. There was an article in Science [exact reference?] on the neurological effects of catnip on cats. It seems to stimulate the same pleasure centers in the feline brain that orgasm does. Most cats "mellow out" and become sleepy and happy, others start acting very kittenish. A small percentage will become possessive of their catnip and may snap or hiss at you.
You can find wild catnip plants in most weedy areas, and harvest the seed. Or you can buy seed from companies like Burpees or Parks or Northrup King -- most garden centers have catnip seed this time of year -- check the "herb" section. Or even seed racks in the grocery and discount stores.
Catnip is easy to grow. You will need to keep the plant itself out of the reach of the cats as catnip-lovers will quickly destroy it. The best strategy is to get some growing, and then pinch and prune it regularly and give the harvested leaves to your cat. Keep it in its own pot, as it will spread rapidly. Cats will tend to dig up transplanted catnip and eat it roots and all, but are much gentler on plants started from seed. The leaves have to be bruised to release the odor, and transplanting seems to be enough bruising...
Nepeta cataria is the common catnip; other Nepeta species have varying amounts of "active ingredient". A good one is Nepeta mussini, a miniature-leaved catnip that makes a good rockgarden plant. Nepeta is a genus of the Lamiaceae (=Labiatae), the mint family. There are about 250 species of catnip, plus a bunch of hybrids between species. Only about 10 are available in this country, though.
You can order seeds from Burpee (215-674-9633) Nepeta cataria B61424 $1.25; N. mussinii B38828 $1.45
Valerian root is an herb with effects very similar to catnip and generally makes cats a bit nuts. It is however not as readily available as catnip and perhaps a bit more potent than catnip.
Catnip and Valerian both act as sedatives on humans.