When you start a wordpress blog you might want to set it up in a folder called blog - or whatever. Your url would look like http://domain.com/Wordpress. This might be because you are trying to build it before you have a domain or maybe you just want it to be at that url.
Now let's say you want to move that install to the root of the domain - so that to access the wordpress install all you do is http://www.domain.com
This is pretty easy.
You may have to wait a bit for the changes to update because of the cache - If you can get to a page while logged in you can manually delete the cache as well.