Definition of the SimulateStaticDocuments feature of TYPO3.

SimulateStaticDocuments (SSD) is the name of a TypoScript command. It is used to convert TYPO3's default URLs with GET parameters (like /index.php?id=123) into pseudo-static URLs (like /products.html).

The simulated static URLs are better readable for human visitors. Also it can improve search engine optimization. A similar, but much more sophisticated, way to convert URLs into speaking URLs is by using the realurl extension.

SimulateStaticDocuments vs. RealURL

SimulateStaticDocuments RealURL
is a setting in the TypoScript template an extension needs to be installed and configured
is easier to configure offers far more possibilities for URL rewriting
can generate XHTML files that are stored physically on the web server no static files can be stored on the web server