Jenkins is the leading open-source automation server, providing a web-based platform for continuous integration and continuous delivery (CI/CD). In production, it orchestrates build, test, and deployment pipelines triggered by SCM changes, scheduled intervals, or manual requests. The architecture is a monolithic Java web application with a plugin-based extension system, using Stapler for URL routing, Jelly/HTML views, and a modern JavaScript frontend bundled with webpack. Key entry points include the central `Jenkins` singleton, a rich set of CLI commands (e.g., `BuildCommand`, `CreateJobCommand`), and a REST-like API. Data flows involve job scheduling via the `Queue`, execution on agents (controller/agent architecture), SCM checkout, build steps, artifact archiving, and result reporting. External dependencies include a servlet container (Jetty), Spring Security (migrating from Acegi), XStream for XML persistence, and numerous libraries for remoting, templating, and UI components. The codebase is highly extensible through `@Extension` points, descriptors, and a well-defined plugin lifecycle.