A task is the sub-item of a User Story. You can break down a User Story into multiple tasks.
Tips: To break down a User Story into tasks, you can take User Story Definition of Done as a checklist.
Time Tracking using Tasks: User Story estimation is a considered as a relative term of guessing work amount needed to finish a User Story. To track, how much time actually team has spent on completing a User Story is done through Tasks.
Time Sheets help user to view accumulative effort per user.
A Task creation dialog.