Search is a module for searching "core" content along with certain registered modules. You put in a word or two and it gives you back matching, relevent results.
You can see the search module in use in the default templates, like on this page. Simply put {search} in your template, where you want the search form to appear. If you want the results of a search to appear on a different page, you can specify this with the parameter resultpage='page alias'.
For more information, see the Search module in the Admin Panel, in the Extensions menu.