Martin Fowler, with his colleague of Thoughtworks, describe Event Notifications as a mechanism to notify other systems of a change in domain. In the case of event notification the source systems doesn't really care much about the response. It typically doesn't expect any answer at all.
Beneficial of event notification is that implies low coupling between systems. The risk however that logical flows start to arise that are hard to see. Eventually this only can be seen from monitoring a live system. Making it hard to debug and test such a flow.
An event notification does not have to carry much data, often just some ID information and a link to the sender is enough.
Source: What do you mean by “Event-Driven”? by Martin Fowler.