The Foundation documentation is located at http://foundation.zurb.com/docs/.

Foundation is really well documented and provides a lot of code samples and examples to use in your own projects. All the components also contain Sass variables that you can use to customize some of the defaults and even build your own. This saves you writing a bunch of override CSS classes.

Each part of the framework is listed on the left-hand side and you can click on what you are looking for. You are taken to a page about that specific part and can read the section's overview, view code samples, working examples, and how to customize that part of the framework. Each section has a pretty good walk through about how to use each piece.

Zurb is constantly updating Foundation, so you should check the change log every once in a while at http://foundation.zurb.com/docs/changelog.html.

If you need documentation on an older version of Foundation, it is at the bottom of the documentation site in the left-hand column. Zurb keeps all the documentation back to Foundation 2. The only reason you will ever need to use Foundation 2 is if you need to support a really, really old version of IE, such as version 7. Foundation never supported IE6, but you will likely never have to worry about that version of IE.