I'm new to SPS and working with version 2 beta. I'm not clear on the
difference between sites and categories.

When I add a category, it creates a new page accessible through a tab at
the top of the main portal page.

Similarly, when I add a site, it creates a page with some content based
on the template that I select. This page is then listed on the sites
page.

So, if I want to create a page with content targeted to a particular
audience, do I add a site or add a category? I don't understand the
relationship between sites and categories.

Can I use categories to grant and restrict user access to content? Can I
use sites to and restrict user access to content?