A rational number is one that can be represented as a ratio of two integers, that is, by one integer divided by another integer. Zero divided by any non-zero integer is zero. Because zero can be represented as the ratio of two integers, zero is a rational number.
Because any integer can be represented by that same integer divided by one, all integers are rational numbers, including positive integers, negative integers and zero. All integral fractions are rational numbers by definition, as are all decimal fractions that terminate, which implies that they're ratios of some power of ten. The same can be said for decimal fractions that eventually repeat, which is characteristic of integer fractions.