An estimating tool in Agile, Planning Poker is a structured game used to reach group consensus while estimating tasks. It is called poker because it does indeed involve a deck of cards.
Game Play: Each card in the deck is inscribed with a number in the Fibonacci sequence, in which each number is equal to the sum of the previous two: 1, 2, 3, 5, 8, 13, 21, etc. Team members each choose a card that represents their best guess as to the difficulty level of the task, and everyone turns over their cards at once. The high and low cards get the floor to argue their cases, and after that, the game is repeated, the theory being that the players’ estimates will converge through rounds of structured discussion. A Project Manager or Scrum Master may serve as moderator.
The game was first created by Agile consultant James Grenning, an original signator to the Agile Manifesto, in 2002, while working with an XP Team. It was later popularized by Agile consultant and Certified Scrum Trainer Mike Cohn, in his book, Agile Estimating and Planning.