The search module lets users search for specific content on your site. You can search both for users and for particular words. When you are on the "content" tab of Search, you will be able to search for words appearing in the default rendering of node content on your site, which would include the default rendering of any CCK fields, Location fields, Taxonomy, etc., as well as comments. When you are on the "users" tab of Search, you will be able to search the user names of registered users on your site, and if you have sufficient permissions, also their email addresses.
In Content search, if you enter more than one search term the search module will look for content that has all the terms you've entered. If instead you want either one term or another, join your terms with "or." If you're looking for an exact phrase, enclose it in quotation marks.
With Advanced Search you can also look for "any of these words" or "this phrase," or both, you can rule out words you don't want, and you can choose content types within which to confine your search.
You can enable the search module on the modules page (administer >> modules in Drupal 7; administer >> site building >> modules in earlier versions of Drupal).