One female cat and her descendents can be responsible for 400,000 kittens in just seven years, who will need good homes, if they are not to become homeless and doomed to a life of misery, hunger, and disease. Each year, millions of cats have to be euthanised, because homes cannot be found for them. It is absolutely imperative that this number is not added to.
A female cat is spayed The operation the removal of her ovaries and uterus (the reproductive organs) . In the male operation is called neutering and involves surgical removal of the testicles. Although neutering is simpler than spaying, neither operation is particularly complicated or risky (although any surgery carries some risk, as does life).
As to the *best* time to spay/neuter, there is much debate going on. Although cats can be spayed/neutered earlier than 6 months of age, studies are still in progress on whether or not such early spaying/neutering is more or less beneficial than waiting till the cat is older than 6 months of age. consult your vet if you are in doubt about when to spay/neuter.