a Composite number means it is a number that has prime factors other then itself and one (1). Prime means one (1) times itself only.
therefore
18 is not prime but composite because 18 = 2 * 3 * 3, it has more than just one prime number in it's base prime factors.
7 is prime because it is 7 = 1 * 7.
16 is composite
11 is prime