Short answers, pulled from the story.
The Atlantic Division currently includes the Boston Celtics, the Brooklyn Nets, the New York Knicks, the Philadelphia 76ers, and the Toronto Raptors. Four of these teams are based on the East Coast of the United States, while the Raptors play in Ontario.
The Atlantic Division was created at the start of the 1970-71 season, when the NBA expanded from 14 to 17 teams by adding the Buffalo Braves, the Cleveland Cavaliers, and the Portland Trail Blazers. The league also split into the Western and Eastern Conferences at that time, with two divisions in each.
The Boston Celtics have won the most Atlantic Division titles, with 26. The Celtics have also won seven NBA championships while playing in the Atlantic Division, more than any other team in the group.
Since the 2021-22 season, the Atlantic Division champion has received the Nat "Sweetwater" Clifton Trophy, a 200 millimeter crystal ball. It is named for Nathaniel Clifton, a Hall of Famer who was one of the first African American players to sign an NBA contract, joining the New York Knicks in 1950.
Ten NBA champions have come from the Atlantic Division. The Boston Celtics account for seven of those titles, while the New York Knicks, the Philadelphia 76ers, and the Toronto Raptors have won one championship each, with the 1972-73 Knicks the only champion that did not also win the division that season.
Several teams have left the Atlantic Division since it formed in 1970. The Buffalo Braves relocated to become the San Diego Clippers, now the Los Angeles Clippers, and joined the Pacific Division in 1978, while the Miami Heat, the Orlando Magic, and the Washington Wizards all left for the Southeast Division in 2004.