What is Jenkins and how does it differ from GitHub Actions?

Jenkins is an open-source, self-hosted CI/CD automation server with over a decade of history and a massive plugin ecosystem (1,800+ plugins). It uses a master/agent (controller/agent) architecture where the master orchestrates jobs and agents execute the actual build steps. Pipelines are defined in a Jenkinsfile (Groovy DSL) checked into the repo. Unlike GitHub Actions (which is a managed cloud service), Jenkins requires you to provision, maintain, update, and secure your own infrastructure. Jenkins offers maximum flexibility and can integrate with virtually any tool through plugins, but the operational overhead is significant. GitHub Actions is simpler to start with (no infrastructure to manage) and is tightly integrated with GitHub, while Jenkins is preferred in organizations that need on-premise CI for security or compliance reasons.