Basic Rules and Gameplay
Ultimate is a non-contact, self-refereed team sport played with a flying disc (or Frisbee™).
Two teams of seven players compete on a playing field about the same length as a football field, but narrower. At each end of the playing field there is an end zone. Each team defends one endzone. They score a goal if one of their players catches the disc in the opposite end zone.
The player with the disc is called the thrower. The thrower may not run with the disc. Instead they move the disc by passing to team-mates in any direction.
The defensive team gets possession of the disc if an offensive team’s throw is not caught by a player of the same team. Then the defensive team becomes the offensive team and can try to score in the opposite end zone.
A complete list of rules can be found here. All rules and diagrams based on international play and taken from WFDF.org
Spirit of the Game
Ultimate relies upon something called Spirit of the Game. SOTG places the responsibility for fair play on every player. There are no referees so the players are solely responsible for following and enforcing the rules, even at World Championships. Competitive play is encouraged, but never at the expense of respect between players, adherence to the rules, and the basic joy of play.
A full explanation of SOTG can be found here. Taken from usaultimate.org
History
"In 1968 Joel Silver introduced his idea of Ultimate Frisbee to the Columbia High School student council in Maplewood New Jersey, USA. The next year, the first game was played between two groups of students. They used a Wham-O Master disc..."
A brief history of how Ultimate started and how it has grown since inception can be found here.
Article credit to WFDF.org