Wikipedia Reference Information
The two round system (also known as the second ballot or runoff voting) is a voting system used to elect a single winner. Under runoff voting, the voter simply casts a single vote for her or his favourite candidate. However, if no candidate receives an absolute majority of votes, then all candidates, except the two with the most votes, are eliminated, and a second round of voting occurs.
Runoff voting is widely used around the world for the election of legislative bodies and directly elected presidents. For example, it is used to elect the French National Assembly and the French President, the presidents of Austria, Brazil, Finland, France, Ghana, Portugal, Romania and for many primary elections in the United States –see: Table of voting systems by nation.
The complete, up-to-date and editable article about Runoff Voting can be found at Wikipedia: Runoff Voting