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 and drag it from the list on the left to someone's stack in the calendar.
Note: Once assigned you can click and drag to move the Task's start time or resize it's length.