A URL rewriter is an Apex Class that inherits the rewriter interface and allows a developer to map site URLs to other URLS, for example you can map `/SomeArticle` to a VisualForce page that takes a specific ID, such as `/apex/ArticleViewer?Id=xyz`, and `/SomeOtherArticle` to `/apex/ArticlesViewer?Id=abc`.

From the documentation:

Sites provides built-in logic that helps you display user-friendly URLs and links to site visitors. Create rules to rewrite URL requests typed into the address bar, launched from bookmarks, or linked from external websites. You can also create rules to rewrite the URLs for links within site pages. URL rewriting not only makes URLs more descriptive and intuitive for users, it allows search engines to better index your site pages.