As technology experts who design systems for flipped classrooms, we are often asked to define the concept. Fortunately for us, the University of Washington has done a great job of summing the concept into an infographic.
In essence, educators who "flip" the classroom have students watch the lecture from home and use the classroom time for activities. Proponents find flipped classroom learning much more dynamic and much more effective. It also allows students to learn at their own pace, as they can rewind and rewatch the lectures on demand.
University of Washington's infographic and additional information on the practice is available online.