Bugzilla is a general-purpose computer bug-tracking tool developed and used by the Mozilla organization.
Being web based and open source has made it the bug tracking tool of choice for many projects, both open source and proprietary.
Bugzilla relies on an installed HTTP server (such as Apache) and a database (usually MySQL) to perform its work. Bugs can be submitted by anybody, and will be assigned to a particular developer. Various status updates for each bug are allowed, together with user notes and bug examples.
The notion of a bug in Bugzilla is very general; for instance, Mozilla.org uses it to track feature requests as well.