kod

Well, it's the best thing to do when you don't want to breed them. If you leave a female cat unfixed, she'll be in heat regularly which is really annoying for the owner because the cats cries a lot, and it's not very fun for the cat either, it's stressful. If a male cat doesn't get fixed, he'll eventually start spraying stuff to leave his scent, and that stinks!

No, if you take care to feed them properly they don't get fat. As for the females, there comes a point after the operation when their metabolism changes so you need to be careful not to feed them too much.
Usually sterilized female cats gets a sort of a pot belly, but it's also kinda big operation, it's the whole uterus that gets removed.
As for the males... Well, our little Felix seems even more energetic now than before. -He even starting playing and running around when I took him home from the animal hospital. So, no. There's no effect there. Still, it's necessary to feed them well because castrated and fat male cats are in the risk of getting urinary infections and stuff.
When they grow older, they obviously tend to calm down a bit and get more lazy, but that applies for all cats.
I read yesterday that if you castrate your cat at an older age, like at age 5 or so, they calm down a lot. That's because they'll stop searching for female cats.