If you are presented with a pile of 28 stones, you will eventually deduce that the pile can be divided into 2 equal piles of 14, 4 equal piles of 7, 7 equal piles of 4, etc. However, if one more stone is added to the pile, creating a total of 29, you can spend as long as you ike, but you will never be able to divide it into equal piles (other than the trivial 29 piles of 1 stone). In this way, we see that 29 is a prime number, whereas 28 is non-prime or composite.
All composites break down uniquely into a product of prime factors: i.e. 28 = 2 × 2 × 7. Note that 2 is the only even prime – all other even numbers are divisible by 2. 1 is neither prime nor composite by convention.