Spaying and Neutering are the common terms for what veterinarians call "surgical sterilization". This is also referred to as "fixing" or just "neutering".
It is the most common sterilizing method in animals. In the United States, most humane societies, animal shelters and rescue groups urge pet owners to have their pets spayed or neutered to prevent the births of unwanted litters, contributing to the overpopulation of animals (http://en.wikipedia.org/wiki/Neutering). If you are wondering if the expense of the surgery is really worth it, the answer is a resounding "Yes". This is a one-time expense that can dramatically improve your pet's quality of life and prevent some frustrations for you (Spaying & Neutering brochure from www.avma.org).