What happens when a cat is spayed?

First, you can't spay a male cat - the proper term for a spay operation is ovariohysterectomy [removal of the ovaries and uterus], and male cats don't have these structures. Males are neutered or castrated, during which the scrotum is plucked free of hairs, surgically opened and the testicles are removed. The vas deferens on each side is tied closed and the testicular vessels are ligated. Generally the surgical incisions are left open to allow for drainage of any inflammatory fluid; they heal up just fine in the vast majority of cases.