Unschooling is a term coined by John Holt who championed education reform in the 1960's. Unschooling is the philosophy of assiting your child's education through exposing them to educational opprotunities in real life and relating important material to your child instead of teaching out of a prepackaged curriculum. It is often called child led education or intrest directed education as well.
An example of unschooling would be your child learn about math, science, history, and language arts through their intrest in Yougioh (the most random thing I could think of). For math they would learn about the thousands of duel monsters out there and the complex math rules of the card game (trust me that some serious math there). The science would come from learning about television and the technology that goes into making a show. Also they would learn about biomes by learning about how different monsters live and do better (in both the game and show) in different environments and then discussing with them how that relates to animals in real life. History would come from learning about the Japanese culture and the history of television, card games and their impact on society, Language arts could come from writing to a Japanese pen pal about anime they have their and the societal differences such as America's censorship of material allowed in Japan.