Ultimate, somethings refereed to as 'Ultimate Frisbee' is a team field sport with many parallels to basketball, football and soccer. The standard outdoor field has a similar layout to football, 70 yards x 40 yards with 25 yard endzones. Players attempt to move the disc downfield by completing passes while the defending team attempts to intercept or block a pass. A point is scored when a team catches the disc in the endzone. Each half starts with a 'pull' similar to the kickoff in football. Play is continuous between points. Whenever a there is a turnover, the defending team becomes the offense and plays on. Contact is limited to incidental play and all players are responsible for making active calls during the game. There are no official referees making active calls, even at the highest level of play. Some games make use of 'observers' who's roles are limited to active line calls (in and out of bounds) and resolving rule violations if players cannot resolve the play quickly.
Check out the members section for more information on Ultimate leagues and programs in your area.