An inbox (sometimes written as "In-box") is a collection point for tasks and things that will generate tasks. The purpose of the inbox itself is not to be well-organized, but to serve as a bounding box for those things which have not yet been organized. They are containers for the mess until it can be sorted through, which should be done on a frequent (daily or weekly at most) basis.
Inbox Zero is getting to empty inbox.
Inboxes can be of multiple forms, and you may have many. If you have multiple, you may want to keep them listed in your Master List.