Originally the site started out as with mostly math and science courses and this is still the most developed subjects but it also contains courses in economics and history and tons of other stuff (There are currently over 3000 lectures available on the site). Each individual lecture is never longer than 10 minutes long so it's easy to sneak one in when you have a few minutes left over. There are also practice courses that help you apply what you have learned that are using all the latest "gamification" methods making it really fun to do them (I like doing it myself when bored). The courses start from the very basics and then build on each other onto increasingly advanced topics.
Kahn Academy started out with Salman Kahn realizing that he was spending a lot of time helping to tutor his relatives and friends and started putting short courses on YouTube. It grew into a very popular channel and in the end led to quit his job in finance as a hedge fund analyst at Connective Capital Management and work on it full time. The Bill and Melissa Gates foundation is an enthusiastic backer of the project and Bill at one time was quoted saying "I'd say we've moved about 160 IQ points from the hedge fund category to the teaching-many-people-in-a-leveraged-way category. It was a good day his wife let him quit his job".