Zero is not a prime number. Zero divided by any other number other than itself, is still zero. It should be noted that since zero is neither positive nor negative, it's not considered a composite number, either.
Prime numbers are numbers that can only be divided by 1 and themselves. An example of a prime number would be 7, as the only whole number it can be divided by is 1 or 7. Any other whole number than can be divided by different whole numbers is called composite. An example of a composite number would be 4, as it can be divided by 1, 2 and 4.