Some web sites use breadcrumbs for navigation. "Breadcrumbs typically appear horizontally across the top of a web page, usually below title bars or headers. They provide links back to each previous page the user navigated through to get to the current page or — in hierarchical site structures — the parent pages of the current one. Breadcrumbs provide a trail for the user to follow back to the starting or entry point."
Google started to detect web pages that use breadcrumb navigation and replaced the URL displayed below the snippet with the web site's structure. "Often, URLs are too long, too short, or too obscure to add useful information. The new text provides useful information about the page." As an added bonus, each hierarchical element links to the corresponding site category, so you can easily navigate to a more general category.
Unfortunately, Google's search results pages no longer look consistent and you'll sometimes have to mouse over a search result's title to find the URL.