Joomla features a component called Redirect Manager. It works in conjunction with a plugin called Redirect, which is enabled by default. The Redirect Manager and plugin keep track of any "page not found" errors that occur when visitors are trying to visit pages in the current domain that have been removed or deleted. In the Redirect Manager screen, these URL errors are listed. This way, you can keep an eye on the old URLs that still attract visitors, but just generate "page not found" errors. For each of these URLs, you can choose to redirect future visitors to the right pages.
This feature can be quite useful, especially if you're migrating to a new site. URLs from your old site are bound to change, resulting in broken links from other sites that still point to expired URLs. Using URL redirection will also help search engine spiders to detect valuable content, instead of just hitting a dead-end error page.