Who was Pythagoras?

Pythagoras was a greek philosopher and mathematician. He is most famous for the theorem named after him - Pythagoras' theorem. It states that in a right triangle the area of the square of the hypontenuse is equal to the sum of the areas of the squares of the catheti. 
The hypotenuse is the side of a right triangle that sits opposite of the right angle. The catheti (or legs) are the sides thet form the right angle.