In the default PrestaShop theme, the categories are listed in a CATEGORIES block. We have just learned how to have a link for categories in the top horizontal menu using the Wiznav module
Some would prefer a theme that will straight away show all the categories that we have in our store in a top menu where all the categories are displayed automatically as horizontal menus.
The next module used can simplify theming. It is the one that displays all the categories within the default categories block, spread into the top menu.
You may find the original discussion at: http://www.PrestaShop.com/forums/viewthread/26478.