What is the deal with cat's and catnip? Why are cats affected by this plant and no other animal?

Catnip is an herb of the mint family. Like all mints, it has aromatic oils on the surface of its leaves. The magic of catnip comes from nepetalacone, one of the chemical constituents of its aromatic oil. When the fragile membranes around the leaves are broken, the scent is released. A pleasure center in the cat's brain is triggered and they go into "cat happiness."
Many adult cats (but not all) enjoy playing and rolling in catnip, and sometimes even licking it (kittens usually are not affected by catnip until they are at least six to nine months old).

Minor point, but my Ferret always enjoyed catnip and, if I remember correctly, many of the large cats are effected.

Thanks for the info.. so ferrets too?

Whenever catnip is mentioned, it always puts me in mind of the 'Southpark' episode when Cartman's cat had his friends round the house while everyone was out, it showed them all getting the catnip out, some were rolling joints with it, others were doing lines on the coffee table and others were lay in the corner, spaced out of their skulls! HA HA HA!

That was a good episode...We have catnip growing in the garden and I used to find the neighbors cat laying in the catnip patch all spaced out all the time, I'd walk up to him and he would be all like "hey man and rolling around!!!