This article is from the Getting A Cat FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Most cats do not belong to any particular breed. These cats are often called "mixed breed" cats. They are also known as "domestic shorthairs" or "domestic longhairs." Domestic shorthairs and longhairs vary tremendously in looks and personality. They come in a wide variety of color patterns and may sometimes closely resemble specific breeds even when they are not. Each one has its own unique personality, regardless of what color it is or how long its hair is.
Domestic shorthairs and longhairs are easy to acquire. In fact, many cats and kittens are killed at animal shelters because there are more cats than there is demand.
Purebred cats are uncommon, estimated at between 1% and 3% of all cats. There are about 40 recognized cat breeds. Each breed consists of a closely related group of cats with similar looks and personality. For example, typical Siamese are slender, active, people-oriented cats that tend to vocalize a lot. Not all Siamese have these characteristics, but most do. A purebred kitten will probably grow up to be typical of its breed in looks and personality; a non-purebred kitten may turn out quite different from what you expect.
Many people are attracted to purebreds because they want a cat with a particular color, size, or hair length. For example, you might be interested in Russian Blues because you like the blue-gray color, or you might be interested in Maine Coons because you want a big shaggy cat. But it's not necessary to buy a purebred to get these physical characteristics. You can find blue-gray cats, or big shaggy cats, or cats of any other size and description, at your local animal shelter. If you're more interested in specific personalities, a purebred might be more predicatable: while any personality type can be found among the non-purebred population, figuring out which one has which may not be as straightforward unless you are looking at adult cats.