Complex numbers include real numbers and imaginary numbers. The unified expression is a+bi, where a is the real part and bi is the imaginary part. When b=0, a+bi represents a real number, when b is not equal to 0, a+bi represents an imaginary number, and when a=0 and b is not equal to 0, a+bi represents a pure imaginary number. I is an imaginary unit, and the square of I is equal to-1.