Task and Story Estimates

There are specific ways to estimate the effort required for a task and story. This is done during the sprint planning meeting.

A task is estimated in terms of person-hours. Note that, it is the effort estimate, and not the estimate of the duration or elapsed time.
A simple way to arrive at an estimate is 'Planning Poker'. Each team member is given a set of poker cards. For a given task, each team member 'votes' for an estimate number, using one poker card in his/her hand. All team members are supposed to vote at the same time, so that, nobody's vote is 'influenced' by someone else's vote. Once all votes are in, the members are required to justify their estimate. A re-voting is done, if required. At some point of time, all estimates 'converge' in a single one.

A Story Estimate (or PB Estimate) is the sum of all the tasks in the story (or Product Backlog). Typically, at the story or PB level, the estimates are expressed as points. So, the sum of the task estimates is divided by 8 (assuming 1 point = 8 person hours). This gives the Story (or PB) Estimate.