Imaginary numbers are numbers that include a unit of the imaginary number i, which equals the square root of -1. There are two types of numbers, real numbers like integers and rational numbers, and imaginary numbers like 3i. Imaginary numbers are used in calculators by changing the mode to include complex numbers with real and imaginary parts. When working with imaginary numbers, you need to follow rules like not having i in the denominator and using conjugate pairs when dividing. Box diagrams are used to multiply terms with real and imaginary parts, while the quadratic formula can be used to solve quadratic equations that produce imaginary number solutions.