In Scrum and Extreme Programming, a unit of work, estimated in hours. During the iteration planning meeting, user stories are decomposed into tasks. In his book, Agile Software Development with Scrum, Ken Schwaber writes, “Tasks should have enough detail so that each task takes roughly four to sixteen hours to finish.”
In practice, many Agile practitioners vary from this formula, by preferring more granular tasks, or more abstract sizing units than hours(e.g., story points). Some do not decompose stories into tasks at all, preferring to add user stories directly to their task board.
Agile Software Development with Scrum by Ken Schwaber and Mike Beedle
Iteration Planning on www.extremeprogramming.org.