N is for neutering. Spaying has the same result, but starts with a female cat.
The main effect of neutering is that the cat cannot reproduce; the cat cannot bring forth kittens into the world. In many cases, if that were all it did, it would have been enough. But there are other benefits.
Neutered cats do not tend to roam as far. Neutered males spray less than intact toms, and it tends to stink less when we do. Toms are not as attracted to spayed females as to females in heat. Female cats who are spayed before their first litter tend to be at a lower risk for mammary cancer (while human females have a lower risk by having children). Males don't get testicular cancer without'em, and have lower risk of prostate disease. Cats who do not get pregnant do not die from complications of kittening, as two of the queens on our farm did. Spayed or neutered cats are less likely to fight other cats, leading to fewer bites and less spread of disease.
When Inky, Truffles, Parker and I were fixed, we went to Protect Every Pet (PEP). The cost was $25 (it has gone up to $30) and rabies shots were $5 (now $10). There was about a six-week waiting period before we could go, and they only had room for two spays that morning. Although PEP treats both cats and dogs, they do not treat both species on the same day. The week we went up, they were only fixing cats. They told our human to make sure we didn't eat anything overnight; he put us in a dog carrier so we did not. Truffles had one piece of kibble as we were being transferred to the cat carriers. Parker and I went up in one carrier (front seat) while Inky and Truffles were in the other (back seat). It was my longest car ride. We left early; about the time that he would have seen us for breakfast on a regular day, we woke up and started talking to him. We were almost there. He dropped us off, and they did their magic. I peed in the carrier as they were putting us back in . They put Parker and Inky in one carrier while I shared the carrier with Truffles. Parker got the front seat again.
The cost is so low because the vets donate their time, and they rely on donations of cash and products from the public. One of the products they need that our human was able to supply was distilled water. They also accept donations from other similar-minded non-profits, such as Athletes for Animals founded by David Backes. It is true that PEP does send the pets home the same day, but they do offer a free follow-up visit the week after the spay surgery.
A4A also has a link to SpayUSA's low cost clinic referral form. Your vet, county animal office, or humane society/SPCA may have more information as well.
(some information from Spay USA)