A story point is a metric used in agile project management and development to determine (or estimate) the difficulty of implementing a given story. In this context, a story is a particular business need assigned to the software development team. Story points are usually expressed according to a numerical range, such as an adaptation of a Fibonacci sequence, or according to a size range from X-S (extra-small) to X-L (extra large).
Elements considered in assigning a story point include the complexity of the story, the number of unknown factors and the potential effort required to implement it.
This Geek and Poke cartoon illustrates how story points are assigned.