Catnip, Nepeta cataria, contains nepetalactone (nepeta lactone), which is what makes Cat Cat go crazy. It also contains some amount of valeric acid. I don't know if this contributes to its relaxing effect or not.
Worldwide, there are over 100 species in the genus Nepeta. One of the most common species (besides Nepeta cataria) is Nepeta mussinni, which is often called catmint, which is confusing, because Nepeta cataria is also called catmint... Hence the importace of botanical names. Nepeta mussinni is less attractive to cats, but looks prettier, with purpleish-blue flowers instead of the whitish-pink of Nepeta cataria. It's usually only about a foot tall, and is grown often as a border for gardens. There's also Nepeta cataria var. citriodora (lemon catnip), which contains higher-than-normal amounts of limonene. Nepeta camphorata is a smaller plant, with white purple-dotted flowers, and bearing camphor-scented leaves. Nepeta parnassica, Greek catnip, is low growing, usually about eighteen inches tall. Its leaves are sweeter than normal catnip, and its flowers are white to pale pink and sometimes blue.
Catnip does make a good tea, and, depending on the strength, can be anywhere from somewhat relaxing (mostly from drinking hot, yummy liquids) to very relaxing. I've heard of people having sucess smoking it, but have only burned enough to know how it smells (sort of funny, like burned rubber, not at all like catnip, but sort of interesting anyways). I've had the strongest effects from snorting it. I know that sounds gross. I don't care, E2 made me a twisted person. Just get some good dried stuff, powder it in your hand, and snort it. Yeah. Like snuff. I wouldn't say it makes you high, but it is quite relaxing, and there's definitely something there. And yeah, I've only snorted it a couple times. I've heard mixing catnip with your bud enhances the feeling, and that mixing it with tobakkey increases the feelings of both.
I grow my own (it's Nepeta cataria). It's easy to grow, just pick up a packet of seeds from any local store, plant in some nice healthy, well drained soil, keep moist, and within two weeks, they should sprout. Pinch them off once they reach their fourth set of leaves (or so), and again whenever it looks like it needs it; catnip can get quite leggy indoors. If you haves cats around, you can put a birdcage over it. Harvest leaves for use as needed, or cut it off down to about six inches when its flowers are in bloom. Hang it upside-down somehere cool, dry, and dark for a couple weeks (or until the leaves crumble easially), strip the leaves, flowers, and buds, put them it into airtight jars, and compost the stems. Or you can freeze it without drying it. It's a perennial in most places, and can become quite prolific if allowed to go to seed outdoors, so be careful!
Meow!