A number that is not prime is called a composite number. Composite numbers can be divided up into smaller whole numbers called factors. A prime number has no factors other than one and itself.
Most integers, or counting numbers, are composite numbers. Primes are comparatively rare. All even numbers are divisible by two, so they are composite numbers. As a number grows in magnitude, the chance that it is divisible by some lesser number increases, so primes become fewer and fewer as magnitude increases. By convention, one is not considered prime. The number two is considered prime because even though it is an even number, it is only divisible by one and itself.