Kittens are little baby cats. They relate to cats the same way puppies do to dogs—when they grow up, they're cats. Kittens are often adorable.
On the other hand, kittens haven't yet learned the cleanliness that makes cats famous. They will do poops and pee all over whatever, and then kind of just let it be. They don't really know how to be clean about it. It's a little gross.
Also, kittens are somewhat mischievous. They like to get into places where they're not supposed to be, and also claw things. Their sharpness is much lesser than the sharpness of adult cats, so they can do less furniture damage, but they still like to wobble around, going "ee, eee," and "snorf, snorf" while they attempt to fight the legs of your furniture and eat plastic.
Nevertheless, kittens are cute as heck. They are tiny and adorable and really great. The main thing you need to be careful about with regard to kittens is that if you take one in and treat it like a member of your family, you might be responsible for it until it dies, which might be 13-20 years. So, be careful not to fall in love with a kitten unless you are in it for the long haul.
Overall, because they're too tiny to do the damage they want to do, we consider kittens to be pretty much not a danger.
