Read "Remainders and Modular Arithmetic", page NT-6 to the top of NT-9. Section 2 and Section 3 are motivational, and you should at a minimum scan over them. These discussions pertain to integers. In considering divisors of zero as you read through these sections, remember that every integer a divides 0, since a * 0 = 0. This also holds for rational, irrational, and real numbers, since w * 0 = 0 for any real number w.
We say that division by zero is undefined. Why do we say that? Consider the following line of reasoning: suppose x is a non-zero number, and when it is divided by zero, the result is the specific number y. It can be proved that, if x / 0 = y, then x = 0 multiplied by y (that is, x = 0 * y). But, then, x = 0 * y = 0, because any number multiplied by 0 is 0. However, we assumed that x was non zero, a contradiction.
If x were 0, then x / 0 = 0 / 0 = y. Again, this implies that 0 = 0 * y. This, in turn, implies that y could be any number. This is, again, a contradiction, because we assumed that y was a specific number.
Mathematical definitions and proofs are specified to adhere to certain rules. One of those rules is that results do not lead to contradictions. Therefore, we say that division by 0 is not defined.
Theorem 1 on page NT-3 states that for any given integer, > or = 2, can be written as the product of prime numbers. Each of these primes will be a divisor of the given integer.