Users in the designer role can add, edit and remove task types. Switch to the designer role and pick tasks on the left hand side.
More complex tasks can have a structure where they have subtasks and/or tasks with checklists on them depending on your requirements.
The task and its subtasks are layouts out in table form where the context menu offers the ability to edit, deactivate or add subtasks or checklist items.
Once you deactivate a task existing instance will be visible to the user but no new instances of this type will be able to be created.