This is the Javadoc for Jenkins core. Multiple versions are available: The latest weekly release, as well as the most recent LTS baselines. Unless a plugin requires very recent functionality, it should generally use one of the LTS baselines for its Jenkins core dependency.
Components Javadoc describes Jenkins components and libraries (currently a partial list of components).
Modules Javadoc describes the latest versions of Jenkins Core modules (Jenkins releases may use earlier versions)
Plugins Javadoc describes the newest releases of all Jenkins plugins, if available.