The Pythagorean Theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. It was discovered by the Greek mathematician Pythagoras around 580 BC and can be used to determine if a triangle is a right triangle or to solve for a missing side. Examples are provided to demonstrate using the theorem to check if a triangle is right by adding the squares of two sides and comparing to the square of the hypotenuse, or to solve for the missing hypotenuse or other side.