Collection points of related tasks to accomplish.
To be organized, you need a place to put the things you need to do. Open Actions Lists are containers for related sets of tasks you need to accomplish. Open Action Lists can exist in many (and multiple simultaneous) forms and locations. Physical places, logical folders, and task management applications can all work well as open action lists.
Your Inbox is just one type of open action list. Task managers are specially designed to be useful for dealing with Open Action Lists.