Wide Layout hides the left navigation and extends across the page, the widest of the three Content Page layouts.
It works best for pages where only the global navigation is needed (since it hides the left navigation), which is a pretty rare instance. It nicely displays the Gallery and Slideshow web parts but is also useful for Shared Content, FAQ and Tabs in certain circumstances. Overall this layout is not recommended for regular pages.
Here's an example of the Wide Layout in edit mode:
Changing the layout of a page is easy -- here's how: Changing a Page Layout.
PLEASE NOTE: This layout is not recommended for most pages as it hides the left navigation.