Lists help you separate your tasks based on the context in which it makes sense to work on them. (i.e. Work or Personal)
Enable/Disable lists in Preferences/Lists.
Change the list you are working on by going to View/Lists
(or by clicking the list name on the toolbar)
(or by keyboard shortcut ^⌥⌘ + the first letter of the list's name)
The method of working on tasks stays exactly the same but it only applies to the tasks in the selected list.
Move the task from one list to another by using the list Popup in the edit view.
Add/remove/edit lists in Preferences/Lists