Folders can be used to store your mail messages. You can move items from one folder to another by checking the message or contact and then select the folder from the Move to... list.
Two types of folder exist:
System folders. System folders cannot be moved, renamed, or deleted. Examples include your Inbox, Sent, and Trash folders.
User-defined folders. You can create folders to organize your mail. Folders you create are displayed in the Folders list in the Overview pane. Top level folder names must be unique. The name cannot be the same as any other top-level folder in your mail or address book folders.