They are schools which are paid for by taxpayers money like all state schools but are not actually run by the local authority....they are free to any one and you don't have to have an entry exam....they can be set up by anyone such as a bunch of parents...businesses etc....the difference in one aspect is that the children of the founders get preference for entry....
Not sure if that covers it but it's the general idea....conceived by the Tories a few years ago...not sure how successful they are...