I believe they are just booking agencies. They work with airlines and hotels and negotiate discounts which they pass (most of) on to their clients. They also earn a commission from the airlines and hotels.
They are not tour operators (at least Expedia is not ... I don t know about Bookings.) Expedia only sells tickets. They don t run tours.More info:http://en.wikipedia.org/wiki/Expediahttp://en.wikipedia.org/wiki/Expedia,_In��