The Watergate scandal is a broad term used today to refer to the series of events that led to the resignation of President Richard Nixon in his second term.

Watergate specifically is a DC hotel and office complex where the Democratic National Committee's headquarters was. Some men broke into DNC headquarters, and Nixon's campaign clumsily attempted to cover up their involvement with this incident. Nixon's dirty tricks campaign and other issues gradually were unraveled through a series of events, most famously the Washington Post's coverage vis a vis the Deep Throat informant (much later revealed as FBI Special Agent W. Mark Felt). There is a lot more to it, but it was the domino that fell that led to huge upheaval in the federal government.

A good starting point is All the President's Men, the book by Washington Post reporters Bob Woodward and Carl Bernstein, or the film of the same name.