Heartbeat is an API that allows logging activity messages to the database. These logged messages are listed up to form a heartbeat stream. There are three built-in streams but developers can register their own. How these messages are getting logged, is fully up to you. Programmers can call the api function for events based on custom criteria. Non-developers and themers can enable the submodule heartbeat_rules so they can trigger events for heartbeat to log the activity. The powerfull module it depends on is rules.