Catnip is an herb in the mint family that 50 percent of cats seem to react to. Catnip grows wild in many places in North America and Europe and the plant can grow to 3 feet high. Once picked and processed for cats it looks quite a lot like tea but much greener. It can also come in a concentrated liquid form. You can buy either form of catnip in most pet stores.
Many cat toys are filled with catnip or have catnip rubbed on them or sprayed on them. The cats that react to catnip rub and lick and knead at the objects, often with incredible vigour. They put on quite a show when toys are sprayed with liquid catnip or rubbed with raw catnip. We have entertained guests on many occasions with this activity.
Interestingly enough it seems like cats are not black or white with regard to if they react to catnip. Our cat Ziglet showed no reaction to catnip for 13 years, while his brother from another mother Zak, always went bonkers for it. Now Zig loves the stuff and reacts to it as well.