Plone search engine allows searching any objects within the Plone site. The peculiarity of Plone's search engine is that it allows advanced searching: by the selected criteria e.g. certain content type(s), or items in certain publication state only, items by certain author(s) only.
Plone search also allows to perform search within current site section by omitting search results from the rest site.
Live Search
Plone's Live Search increases the effectiveness of Plone's search engine by simplifying the whole search process. As you type in letters for a search - possible matches appear below the search box. Thus, search results appear instantly on a search widget above the current page and you can navigate between them with your keyboard. So you don't need to enter a search term, then wait for results to appear on the new page.
This live feedback on searching makes finding things much faster much more effective.