Public Events are random PvE activities that spawn in patrol zones, allowing multiple random players to meet and complete a single objective like defending an object or beating a powerful enemy. Public Events are tracked on your map and can be fast traveled to.
Secret Public Events are similar to Public Events but they are not tracked on map and cannot be fast traveled to. They are also smaller in scale and do not have Heroic completion.
Heroic Public Events
Heroic Public Events are the heroic versions of Public Events. The heroic mode can be triggered by completing the optional objective. This will increase the difficulty of the public event, but if defeated you will be granted bonus rewards.