The Admin Panel Structure

Data - contains details of everything that is generated from the front end (the customer's view) of the site. For example if you are running a shop you will see your orders. On most sites there are enquiry forms and the results of the forms can also be found in this section.
Content - this is where your pages can be found, e.g. About Us, Contact Us, Home etc.
Structure - where the CMS deals with categories, menus, widgets and layers
Tools - Manage files, images and build forms etc.
Admin - Everything else that doesn't fit above. For example email templates, locked content etc.