"Planning Poker, also called Scrum poker, is a consensus-based technique for estimating, mostly used to estimate effort or relative size of user stories in software development. It is a variation of the Wideband Delphi method. It is most commonly used in agile software development, in particular the Extreme Programming methodology. The method was first described by James Grenning in 2002 and later popularized by Mike Cohn in the book Agile Estimating and Planning." http://en.wikipedia.org/wiki/Planning_poker
Ok. So basically, you've got a task, or a UserStory, depending on your approach and you ask everyone for a ball park esitmate.
People vote by showing a card.
I worked with a team that used real cards once, and there's a certain value in the Scissors, Paper, Rock style of playing this game where everyone estimates simultaneously.
But with or without actual cards, for me the real value of this game is in situations where one developer knows how to do something and other's don't.
Take a job "stop the UL/LI menus from wrapping"
The ETL SME holds up his card which basically reads "not this century" and someone else on the team says "5 minutes".
It's good for the ProductOwner to get a range of quotes, and it helps everyone establish where strengths lie.