Wordpress - Looking for WordPress System Diagrams

^{More than an Answer, this is a research and a compilation. Database Description already in Damien answer.}

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A Google search reveals this in WordPress forums:

UML documentation of WordPress
As far as I know, that's all we have. If you want to write one, I'm sure it'd be welcomed :)
(Ipstenu, referring to the Database Description)

UML of WordPress MU?
There will likely never be UML diagrams of WordPress because it's not an OO system. Some parts are OO-like, but overall it's whatever that particular code contributor made work. There's no map, just code.

And IMO, the point of UML is that it's done in the architecting phase. A project like WordPress would be hard-pressed to have this since it's a fluid integration of independent developers contributing to the core.
[...]

Welcome to open source.

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Note: the Template Hierarchy of WikiMedia in the OP Question differs from the one in the Codex

^{click to enlarge}

But, Chip Bennet has an extended one :)

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I thought this Question was a duplicate of the following (but mods said it's not):
Where can I find a real architecture document on Wordpress?

In it, Mike Schinkel states:

These generally are not some the open-source WordPress community focuses on doing.

One answer points to an old article with this diagram:

And the other to this Wordpress 3.0 Program Flow (PDF)

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Rarst answer to this Question ( When should you use WP_Query vs query_posts() vs get_posts()? ) has a nice overview of WP_Query:

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Another from Rarst on core load:

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And finally, a totally unrelated Question about performance ( Refactoring Wordpress to improve memory performance ), but with very nice graphs :)

If you are looking for the database schema, there is a version on the Codex:

^{click to enlarge}

The image you referenced in your post is the template hierarchy most commonly used by theme developers.