Whole numbers which are not a prime number are called a composite number. These numbers can be divided into a product of two, or more, lower whole numbers. $12 = 3 \cdot 4.$ The procedure can then be repeated until $12=3 \cdot 2\cdot 2.$ All composite numbers can be factored into primes in only one way. This relationship ties together all whole numbers in a fundamental manner which is why it is called the Fundamental Theorem of Arithmetic.