-13 is an integer. A natural number is a whole positive number, such as 21 or 174. Whole numbers don't include decimals, fractions, or negative numbers. An integer is any number that isn't a decimal or fraction. A rational number is a number that ends or repeats forever, such as 6.4 (with the four repeating forever) or 6.625. And irrational numbers are numbers that continue forever without repeating or ending, like 8.2546843263...