Planning poker is when people use cards with numbers from the Fibonacci sequence on them to try to rate the difficulty of a task. It is designed to be quick and to avoid the halo effect of someone seeming like they know what they are talking about and everyone thinking themselves wrong. Each person lays down a card with one of the numbers and as long as they are all within one of each other they are averaged. If they are not, the high and the low each explain why they think theirs is correct and the cards are played again and averaged.
The definition of done refers to what needs to be accomplished for a task or story to be considered complete and moved out of in progress. This is critical, because if definitions of done vary, one team member can think a task is done, mark it as such, and move on to the next one. This is a problem because a task that isn't done to the set specs may not work when it is needed.