Integers include natural numbers, their opposites (negative numbers), and zero. Positive integers are natural numbers and lie to the right of zero on the number line, while negative integers are the opposites of natural numbers and lie to the left of zero. The key properties of integers are that the sum or product of two integers of the same sign is an integer, while the sum or product of two integers of different signs is an integer of the opposite sign. Basic integer operations like addition, subtraction, multiplication and division follow predictable rules based on the signs of the integers.