bear with me, i'm on the other side of the world so i just hope that our math terminologies are the same.

topsquark's post is very helpful, i didn't kow that first part! basically a prime number is something that has only two factors, being 1 and itself. our lovely mathematicians are always quarreling over whether or not 1 is prime, but when i last checked, one was not prime. just check with your teacher though.