When you create a task you have the option to save it and mark it as completed immediately - which is very helpful if you are just trying to create a record of some work you have done for a client or internally for reference later.
You also have the option to leave it as an active task and then mark it as completed later.
Either way, once you have marked the task as completed it will disappear from the default active view on the Tasks page.
To see a list of all your paused or completed tasks simply click on the 'Active' filter and change it to 'Paused' or 'Completed'. You can also optionally set a date range to filter it to tasks within a specific timeframe.
This is very handy if you want to create a monthly reports showing tasks you have completed for a client in the past month.