Catnip is an herb. You can actually grow catnip grass in an herb garden, and a lot of people do this either to distract their cats from their other plants, or just to let their cats nibble when the urge strikes.
I'm not sure of the actual physical properties of the herb, but it does affect cats in different ways, some not at all. Their reaction to it is hereditary, so if mom cat doesn't react to it, more than likely baby cats won't either. Some get real "mellow" and others real active. Others react aggresively.
As a sidenote, I was in GNC the other day and saw Catnip tea. It is supposed to be real relaxing for humans, but I could just imagine trying to fight off the cats to drink tea! :tounge2: