The Bugzilla Search page is is the interface where you can find any bug report, comment, or patch currently in the Bugzilla system. You can play with it here: http://landfill.bugzilla.org/bugzilla-2.18-branch/query.cgi.
The Search page has controls for selecting different possible values for all of the fields in a bug, as described above. For some fields, multiple values can be selected. In those cases, Bugzilla returns bugs where the content of the field matches any one of the selected values. If none is selected, then the field can take any value.
Once you've run a search, you can save it as a Saved Search, which appears in the page footer.
Highly advanced querying is done using Boolean Charts. See the Boolean Charts help link on the Search page for more information.