A TiddlyWiki is an html file (a webpage) stuffed with special code. Think of TiddlyWiki as a bunch of 3x5 cards or scraps of paper all linked and interconnected to each other by hyperlinks and tags.
Rather than scrolling up and down a long web page or word processing document, you open small chunks of information ("microcontent") written in boxes called "tiddlers."
Tiddlers are linked to each other in different ways:
A good introductory guide to TiddlyWiki for the uninitiated. It's written for non-techies and explains what TW is good for and why you'd want to use it.