The Watergate scandal happened when United States President Richard Nixon, a Republican, was tied to a crime in which former FBI and CIA agents broke into the offices of the Democratic Party and George McGovern (the Presidential candidate). Nixon's helpers listened to phone lines and secret papers were stolen. This was against the law.
Nixon chose to resign from presidency on August 9, 1974 because he didn't want to be impeached (this means that he may be removed from presidency with a certain decision of the U.S. Congress). So, the U.S. Congress could not impeach him. After this, Gerald Ford, his vice-president, became the President. Ford later forgave and pardoned Nixon for all of his crimes. The name "Watergate" comes from the hotel in Washington, D.C. where the first crime took place and is often associated with political scandals.