A circus is most commonly a traveling company of performers that may include acrobats, clowns, trained animals, trapeze acts, hula hoopers and other fun acts and the word also describes the performance that they give. A circus is held in an oval or circular arena with tiered seating around its edge; in the case of traveling circuses this location is most often a large tent.
The circus originated in Ancient Rome, where the circus was an open-air stadium where chariot and horse races and other public exhibitions were held. Briton Philip Astley is thought of as the father of the modern circus, establishing permanent and travelling circuses in Britain and Europe in the late 18th century. You can find more information at Wikipedia