Khan Academy offers video lessons and instructional exercises covering a variety of subjects. While originally focusing just on mathematics lessons, Khan Academy has expanded to host video lessons on physics, chemistry, economics, history and many other subjects. Khan Academy provides all of its services at no charge, as of 2015.
In addition to the videos, the site provides interactive exercises for a number of subjects. The exercises present the user with a problem or question that relates to the video she most recently watched. When she answers correctly, the exercise presents her with another question, and she makes progress toward completing the exercise. When the user answers incorrectly, a step-by-step guide for solving the problem or answering the question appears.
Khan Academy has a reward system in place to encourage its users to continue learning. Users earn points for watching a certain number of minutes on a particular subject, for watching all the videos on a subject and for successfully completing the site's interactive exercises.
Khan Academy started after Salman Khan, the site's founder, posted the videos he made tutoring a family member in mathematics to YouTube. The videos proved extremely popular, prompting Khan to start the Khan Academy site. While the team of educators at Khan Academy has since expanded significantly, Salman Khan continues to create a large number of videos himself.