The Two Column Layout offers left navigation and a fairly wide Main Content area. It is great for regular content that looks better in a wider format, such as wide pictures, tables, and certain web parts such as FAQ, Shared Site Content, and Gallery.
It offers a nice wide look and does not have a right sidebar (unlike the Three Column Layout).
Here is an example of the Two Column Layout in edit mode:
Changing the layout of a page is easy -- here's how: Changing a Page Layout.