Integers include whole numbers and their opposites. The opposite of a whole number is the negative of that number. The number 0 is also considered an integer, but 0 is the opposite of itself.

0, 1, 2, 3, 4 and -1, -2, -3, -4

It may be easier to understand integers if you see them in order.

... -4, -3, -2, -1, 0 1, 2, 3, 4 ...

Note that the three dots on either side indicate that integers continue in either direction.