Planning Poker is an Agile practice that harnesses the “wisdom of the crowd”. The goal of planning poker is not to derive an exact and accurate estimate that would stand all future scrutiny, but rather, to obtain a group estimate in a fast, cost effective and collaborative way.

Estimating and planning are critical to the success of any software project, also in the case of distributed agile development. Research has acknowledged that conventional agile methods need to be adjusted when applied in distributed contexts. However, we argue that also new tools are needed for enabling effective distributed agile practices.
This article presents eConference3P, a tool for supporting distributed agile teams who applies the planning poker technique to perform collaborative user story estimation. The planning poker technique builds on the combination of multiple expert opinions, represented using the visual metaphor …

Mike Cohn wrote an interesting post where he discusses he allows or even encourages to estimate with story points as large as 20, 40, and 100. He explains that they are useful when you need first and not necessarily precise estimate of the general size of a new project being considered.