OpenXML spreadsheet structure overview?

There's load of information, but I can't seem to find a simple and clear overview of the structure of an OpenXml spreadsheet document (and I'm not the only one, you'll find many messages posted about this)...
What 'are' WorksheetParts, Sheets, SheetData etc exactly, and how do they fit together to form 'the structure' of a spreadsheet?

Anyone knows of such an overview?

The structure is quite complex if you haven't seen it before.  Usually the easiest way to get familiar with it is to use the OpenXML SDK v2.0's Productivity tool - create a simple xlsx and then have a look at it.

Anything more specific you want to know I'm sure we can give you some help :)