Khan academy

video to explain the principles by the founder of the Academy, Salman Khan.
http://www.khanacademy.org/
Scroll down and choose the lecture and exercise you need  most.
A video to explain the idea behind the 'thing' (by the founder of the Academy)
http://www.ted.com/talks/salman_khan_let_s_use_video_to_reinvent_education.html