Google Sites Tutorial - Horizontal Navigation and Drop Down Boxes
Horizontal Navigation is more appealing because we process information best that way. It makes sense, we read from left to right. Another benefit from this feature is that it eliminates the need for a sidebar. While sidebars have their place in web design it is better to implement it on your terms, not because you have to for navigation purposes.
Also, horizontal navigation enables the use of dropdown boxes. These are links that appear when the category is hovered over. For instance, in my bakery website example. I have a page called Baked Goods, the sub-pages of which will be scones, muffins, cupcakes, etc. Those sub-pages, which you designate in Manage Site > Site Layout > Change Layout > Horizontal Navigation using the left and right arrows, are a nice option for less cluttered navigation.
For examples of horizontal navigation checkout: www.kirksvillehousingauthority.com, or www.pancakecitykirksville.com.
For a walkthrough watch this video below.
For more information Google has posted about this as well: http://googledocs.blogspot.com/2010/10/tips-tricks-making-most-of-your-sidebar.html