General information about interfaces: Listener
Available implementations: Implementations of listeners
Usage of Jagger services in listeners: Relation of listeners and Jagger services
How to customize: Custom listeners

General info: Listeners give possibility to provide some user actions during test flow defined by XML configuration
User can implement custom listener and Jagger will trigger it on some events (f.e. test suit start/stop, test group start/stop, test start/run/stop)