A prime number is a number which can't be divided exactly by anything but itself and 1. It must be a whole number (an integer) and greater than zero (positive).
For example, 2, 3, 5, and 7 are all prime. But 4 is not, because it can be divided by 2.
Any whole, positive number which is not prime is called a composite number. So, 4 is a composite number.
1 is not a prime number.
Prime numbers go on for ever. There is no limit to how many there are. If you kept counting for ever, you would keep on finding new prime numbers.