Up until a few years a few years ago most WordPress themes had a fixed layout.
So if the primary navigation was on the right and the content on the left, then it stayed that way unless you manually edited the template files.
However it is becoming increasing common for newer templates to be more flexible . Even the default 2011 template has several layout options (see below):
Not only do many modern themes have different layout options they also have additional widget areas and the ability to change the colours,fonts, hide portions like the header graphic, and much more; all without having to resort to editing the template files.
When choosing your theme take a look to see how flexible it is as the more flexible it is the less chance you will need to change it for a new one in the future.
