The Common Event Infrastructure is a core part of the runtime that WebSphere ESB and WebSphere Process Server are based on. The CEI runtime allows you to generate business or IT level events and persistently store then for consumption by monitoring tools such as WebSphere Business Monitor. The event format is defined by the Common Base Event model.
Currently in WebSphere ESB 6.0.1 you can generate CEI events when exports and imports are invoked. With the Event Emitter primitive in 6.0.2 you will be able to generate your own events from within a mediation flow, which can embed any information from within the Service Message Object.
A typical use of the Event Emitter primitive would be in conjunction with the Message Filter primitive to generate a business event when an exceptional situation is discovered based on the content of the message being mediated.