Short answers, pulled from the story.
Deep Throat was Mark Felt, who served as Deputy Director of the FBI during the Watergate scandal. He was the anonymous source who gave reporter Bob Woodward key details about the Nixon administration's involvement starting in 1972. Felt revealed his own identity through an attorney on the 31st of May 2005.
Washington Post editors, including managing editor Howard Simons, coined the nickname Deep Throat. It referred both to the term deep background, used for anonymous sourcing, and to a widely publicized 1972 pornographic film with the same title.
Woodward signaled for a meeting by moving a flowerpot with a red flag on his apartment balcony. Deep Throat requested meetings by marking page 20 of Woodward's copy of The New York Times, circling the page number and drawing clock hands to show the hour. They typically met around 2 a.m. on the bottom level of an underground garage near the Key Bridge in Rosslyn.
Felt's family described him as acting for moral and patriotic reasons, calling him an American hero. Woodward's book The Secret Man suggests Felt, a loyalist of J. Edgar Hoover, was also angered when L. Patrick Gray, an outsider with no law enforcement background, was named FBI Director over him after Hoover's death.
Felt confirmed his identity through an article published in Vanity Fair on the 31st of May 2005, written by attorney John D. O'Connor. Woodward, Carl Bernstein, and former Washington Post editor Benjamin C. Bradlee confirmed the claim that same day.
Woodward and Bernstein always maintained that Deep Throat was a single person, Mark Felt. Others, including literary agent David Obst and Ed Gray, author of In Nixon's Web, argued the character was a composite invented or embellished from multiple sources.