Bugzilla is one example of a class of programs called "Defect Tracking Systems", or, more commonly, "Bug-Tracking Systems". Defect Tracking Systems allow individual or groups of developers to keep track of outstanding bugs in their product effectively. At the time Bugzilla was originally written, as a port from Netscape Communications' "Bugsplat!" program to Perl from TCL, there were very few competitors in the market for bug-tracking software. Most commercial defect-tracking software vendors at the time charged enormous licensing fees. Bugzilla quickly became a favorite of the open-source crowd (with its genesis in the open-source browser project, Mozilla) and is now the de-facto standard defect-tracking system against which all others are measured.
Bugzilla has matured immensely, and now boasts many advanced features. These include:
integrated, product-based granular security schema
inter-bug dependencies and dependency graphing
advanced reporting capabilities
a robust, stable RDBMS back-end
extensive configurability
a very well-understood and well-thought-out natural bug resolution protocol
email, XML, and HTTP APIs
integration with several automated software configuration management systems
too many more features to list
Despite its current robustness and popularity, however, Bugzilla faces some near-term challenges, such as reliance on a single database, a lack of abstraction of the user interface and program logic, verbose email bug notifications, a powerful but daunting query interface, little reporting configurability, problems with extremely large queries, some unsupportable bug resolution options, no internationalization, and dependence on some nonstandard libraries.
Despite these small problems, Bugzilla is very hard to beat. It is under very active development to address the current issues, and a long-awaited overhaul in the form of Bugzilla 3.0 is expected sometime later this year.