It is both!
Straight out of the box, without any modifications, you can use TiddlyWiki as a "simple note organizing tool" to create tiddler content with basic formatting syntax, add a few keyword tags, and some links to other tiddlers using [[...]] and embedded images using {{...}}. If that's all you really need, then TiddlyWiki is very quick and easy to learn.
Of course, TiddlyWiki can also be used as a "platform for complex applications", and that's where knowledge of macros, filters, widgets, themes, etc., comes in handy and really shows off the power of TiddlyWiki. Sure, to really "get under the hood" and tinker, there is a bit of a learning curve. Fortunately, however, you don't need to learn it all at once just to make a few customizations and apply a few macros and filters here and there.
enjoy,
-e
Eric Shulman
ELS Design Studios
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