Domain events track changes to your domain data model (if you are not familiar with these concepts, please review the Architectural Patterns page first). The library addresses two aspects relating to domain events -- declaring the events themselves, and handling the events.
A domain event is just a simple data-only class (POCO) which inherits from the library's DomainEventBase abstract class. The properties of the class should represent only the data which changed. The base class does not require implementing anything in the inheriting class. It just adds a few properties that associates the event with the model instance (the unique Id value), the version of the model (called an ETag), and a timestamp.
Each domain data model needs a domain event handler class which implements the IDomainEventHandler<TDomainModelRoot> interface. The interface requires a DomainModelState property of the domain model root type, and a void Apply method which accepts a StreamInitialized domain event argument (although normally the event handler doesn't do anything in response to that event).
From there, you should add a void Apply method for each and every domain event defined for your model.
Before an Apply method is invoked, the DomainModelState property is updated to match the manager's most recently copy of the domain event model. The Apply method should alter the data according to the domain event and the various properties it contains.
The DomainModelState is transient and must not be used for any other purpose. After the Apply method exits, the manager will reset the property to null.
Domain event handlers must be registered at application startup (see the Domain Event Handlers Service topic for more information).