Google Sites Tutorials for ePortfolios
Site categories, or "tags" are keywords words describe your site. Example site categories would be "Portfolio" or "Education." Tags are good to have so visitors can quickly get a feel of the function of your site. Click onto More options to add site categories
On the other hand, a site description describes the overall purpose of your Google Site. Descriptions should be no longer than 160 characters. Click onto More options to add a site description.