On the Settings->General screen in a single site installation of WordPress, there are two fields named "WordPress Address (URL)" and "Site Address (URL)". They are important settings, since they control where WordPress thinks your site is located. They control the display of the URL in the admin section of your page, as well as the front end, and are used throughout the WordPress code.
The "Site Address (URL)" setting is the address you want people to type in their browser to reach your WordPress blog.
The "WordPress Address (URL)" setting is the address where your WordPress core files reside.