Mathematics Illuminated is a thirteen-part series for adult learners and high school teachers. The series explores major themes in the field of mathematics, from humankind's earliest study of prime numbers, to the cutting-edge mathematics used to reveal the shape of the universe.
A good site to show students. If they are at home (and have a computer) and don't understand some homework assignments, they can go to this site to get examples and definitions. In high school they can use it for Algebra, Calc I, and Calc II