Story points are probably the most commonly used estimating unit among agile teams today.  The name is derived from agile teams commonly expressing requirements as “user stories”.  A story point is a relative estimate of the "size" of the work compared to other work on the project. So, a 10-point user story is expected to take twice as long as a 5-point user story.

Estimates are not quantified but implicitly include the team's collective understanding of the complexity, risk and time for the work being estimated.  A story point is unique to the team and cannot be compared with a story point on another team.  It can be counter productive to map story points to their absolute time equivalent and the accepted technique for predicting time to completion is the team's velocity.