The best way to think of Tasks is to see them as the bit of a Todo that you actually assign to someone. It's therefore possible to have many tasks within the same Todo that are assigned to different people in your team.
The blocks that you see on the calendar are all tasks. If you open the project panel from the Calendar Toolbar and select a project you'll see tasks listed within each todo.
You can add new tasks directly from the project panel here by clicking + Assign to person link within the Todo.
Alternatively, to assign the whole Todo you can click the assign icon then select the person you want to assign it to.
It's useful to think of Tasks as children of a Todo so you can have many child Tasks under the same parent Todo.
If you have more than one task under the same Todo when you update the Due Date of that Task it will NOT update the Due Date of the parent Todo. Workstack will only be able to match Task and Todo Due Dates when there is only one task under a Todo.
Once assigned you can click and drag to move the Task's start time or resize it's length.