The Modern Olympic Games is an international multi-sport event subdivided into summer and winter sporting events.  The summer and winter games are each held every four years (an Olympiad). Until 1992, they were both held in the same year. Since then, they have been separated two years apart.  This Wikipedia article on the Olympics discusses the modern Olympics in decent detail.

The Ancient Olympic Games first started in Olympia, Greece, a sanctuary site for the Greek gods near the towns of Elis and Pisa (both in Elis on the peninsula of Peloponnesos). The Sanctuary of Zeus in Olympia housed a 12 meter high statue in ivory and gold of Zeus, the father of the Greek gods, sculpted by Phidias.

The Olympic Games were held in four year intervals, and later the Greek method of counting the years even referred to these Games, using the term Olympiad for the period between two Games. The games were to some extent "international", in the sense that they included athletes from the various Greek city-states. Additionally, participants eventually came from Greek colonies as well, extending the range of the games to far shores of the Mediterranean and of the Black Sea.

The winner of an Olympic event was awarded an olive branch, and was often received with much honour throughout Greece and especially in his home town. 

Archaeologists believe that wars were halted between the city-states of Greece so that the athletes as well as the spectators of the Olympics could get there safely.

Background on the modern and ancient Olympics courtesy Wikipedia.org, subject to the GNU Free Documentation License.