Spaying is the process of removing the ovaries and, usually, the uterus, from a female animal in order to prevent the animal from breeding. Many animals, including cats, are spayed. Spaying does cause some changes in behaviour.
Hormones send signals to the brain, initiating certain behaviours such as going into heat and urine marking. Spaying reduces a cat's hormone levels by removing the ovaries, thereby influencing its behaviour.
Heat, or oestrus, is the sexually receptive phase of a female cat. During heat, cats make loud noise, spray urine and exhibit restless behaviour. Spaying removes the possibility of a cat going into oestrus.
Cats normally do not experience personality changes after spaying. However, the cat may become more lethargic and less "playful." Also, the cat's metabolism will slow down.
There are behavioural benefits to having a cat spayed. For instance, the cat will not go into heat, so she will make less noise. Also, the cat will be less likely to roam.
One common misconception is that a cat will always gain weight after being spayed. Although a cat's metabolism will slow down, weight gain can be controlled by switching the cat to a low-calorie diet.