Real numbers are all rational and irrational numbers which include whole numbers, repeating decimals and non-repeating decimals. Non-real numbers are known as "imaginary" numbers. The square root of negative numbers, which will come up as an "error" on most calculators are imaginary numbers. The square root of -1, is known as "i" and imaginary numbers can be part of the complex number which has a real an imaginary part. And example of a complex number is 2 + i.