Like Issues, To-Dos differ from tasks in that they offer a different structure outside of the formal Project to Task hierarchy. They can also be requests from other team members for assistance, as opposed to official task assignments by a project manager. However, the most common use of To-Dos is to add depth to their Task or Project work load as things to get done within a Task/Project assigned to you.
To-Dos vary as widely as your imagination
Some teams ask their users to use To-Dos like sticky notes all over Project Insight. You can add them to Projects, Tasks, Issues, stand alone in folders, and more. The best practice for To-Dos, however is to keep them in the structure of a Project/Task or Issue rather than letting them stand alone as their own work item.
If a longer task requires multiple steps, you can use multiple To-Dos to denote how far you have completed the task.
Anyone can assign To-Dos to another resource in Project Insight, so ask for help completing a task.