Is 18 prime or composite?
Composite; it's even.
Prime numbers are numbers that only have one factor other than 1, that being the number itself. The number 18 can be factored into 2 * 3 * 3, or 2 * 3². This is known as the number's prime factorization. The number 17, on the other hand, can not be broken down any further into smaller prime factors, and so 17 is prime.
While it isn't always easy to see if a number is prime just at a glance, it can be easy to tell if some numbers are composite. One trick is to see if the number is even (and greater than 2). If it is, then it must have 2 as a factor, and that means it is composite. The number 18 is even (and greater than 2), and so it is composite.
Similar tricks exist to tell if numbers are multiples of 3, 4, 5, etc. To tell if a number is a multiple of 3, simply add the digits together to get a new number. (In the case of 18, we get 1 + 8 = 9.) If this new number is bigger than 9, add the digits again, until you get a sum that's less than 10. If the final sum is a multiple of 3 (i.e. 3, 6, or 9), then the original number is a multiple of 3. For 18, we got 1 + 8 = 9, and since 9 is a multiple of 3, 18 is too.