Tasks are associated with checklists and provide a very easy method to track step-by-step the completion of items on a project. Tasks can be assigned directly to users and they can in turn track their time on that individual task.

Notice in the above image that you can set the duedate for the task as well as notify the assignees upon saving a task.

Clicking on the clock will give you a popup as shown below where you can enter the time spent on a particular task. The time will add to previous times you've entered on the same task.