Teams determine the best way to execute a clearly defined scope of work. They determine their roles, structure, and tasks on an on-going basis.
Imagine that you and your small team have the mission of creating a marketing strategy for a new product. You have absolutely zero oversight or restrictions on the project and it needs to be done in three weeks. What would you do? Who would you need in the room to accomplish this task? How would you distribute the workload?
The people closest to the problem have the best information necessary to accomplish the task. A self-organizing team has the freedom to decide how the work gets done and who completes which tasks.