The search block is a very basic site search that you can configure to match your content.

• Search Title - This is displayed above the site each form
• Search Button Text - This is the copy displayed in the form button next to the search field.
• Search Within Path - This lets you limit your search results. Everywhere will search pages from across your whole site. Beneath this page limits the results to only this part of the site tree and below, while Beneath another page lets you pick which area of the site to search by hand.
• Results page - the search block displays results in the same area it displays the form. If you're adding search to a sidebar or header navigation, this is a problem. You can create a blank page somewhere else with a search block on it, and use it as a search results page. Simply pick that page you've setup as the target for Post to Another Page Elsewhere. The results will display on that new page wherever you put the empty search block.

NOTE: This is a simple search; it's not Google or the Intelligent Search on your toolbar. We do a simple substring search in name, description, and any blocks that have searchable content in areas that have been marked searchable. For more information on areas that are searchable, you should check out Dashboard > Sitemap > Page Search > Manage Index.
This search isn't particularly brilliant about things like plural words, multi-word terms, locality, etc. It also doesn't like searching for words less than 4 characters long. It can be quite powerful if you're willing to tinker with it, but it is not as graceful as google.