Just like a normal browser, Safari on the iPhone offers URL suggestions as you type in a URL. The suggestions are based on the sites you have visited so it gets larger as you visit more sites. Over time this list can get pretty large. So large that it becomes more trouble than it's worth. If you've reached the point where this list is too larg for comfort you can easily clear it out by tapping the book icon in the lower right part of the page when Safari is open. Select Clear from the menu and the list of URL suggestions will be cleared from your iPhone.