Most cats love catnip, each cat has it's own little way of reacting to it, some cats will rub themselves all over it, and others with spend all day trying to eat it. My cat insists on licking the area of catnip until there is nothing left.
Nobody particularly knows why cats go so mad over catnip, it could be the scent of the plant, there may be a plant hormone similar to cat hormones, or it could act like a drug to your cat, no one cat be totally sure, as each cat will act in its own way.
Catnip has a common name of "cat mint", and it comes from the mint family. It is actually edible by humans, you can put the pretty purple flowers into your salad in the spring (if you've got any left and the neighbourhood cats haven't got there before you). The Latin name for catnip is Nepeta cataria, and originally was grown as a weed in Europe, but now the plant grows all over the world, since it's commercial value in cat related products has been realised.
In some stores you'll be able to find catnip in bottle form as well as in a packet, this is because the bit that drives your cat crazy comes from the oil in the leaves. This is called Nepetalactone, which can be extracted in the same way your favourite essential oils are, normally by steam extraction.
There are plenty of other uses of Cat Mint, you can use it as normal mint, and infuse on leaves in hot water as a tea, this will help you sleep and aid digestion. If you grow it near your door it will also keep those nasty bugs out of your house.
Catnip doesn't affect all cats, some are not bothered by it at all. It will affect about half of the cats in the world. This is because there is a gene that is inherited that will determine whether or not your cat will go mad for cat nip. You will not be able to tell whether your cat likes catnip until it is about 4 months old, as they do not show any affect before this. Also when your cat gets older (an elderly cat), it will show less and less interest in catnip. You can almost guarantee if your cat's relatives were from a region that was originally native to catnip, then they will be effected, where as those from other parts of the world probably will not be.
Catnip is great for training your cat. If you have a sofa scratching cat, you can encourage you cat to use a scratching post, by placing it near the area it is scratching and spraying some catnip on it, you cat will then become far more interested in the post rather than the sofa.
If you have a cat that simply loves catnip, why not grow your own. Visit your local garden centre and simply look for the Cat Mint plant or seeds. It's very easy to grow and can take over your whole garden (it's very invasive) if it's not contained. So try to grow it in a container, buying a small plant is better than growing from seed, as your cat may be far too interested in the seedlings to let them grow to maturity. These won't set you back much and due to the spreading nature of the plant you'll probably only ever have to buy one. This will save you loads of money, as in the shops catnip can be very overpriced. Simply cut off a few leaves at a time when you need it. Or you can cut off whole stems at the end of the summer and hang them to dry in a warm and dry place (like and airing cupboard), and then crush them up and keep them in a jar, as long as there is a lid on the jar they will keep indefinitely.
Don't go mad with catnip, as your cat will loose it's sensitivity to it. So give is as a treat, and your cat will be far more appreciative.