Plug-in Message is the triggering event, such as Update or Create. There are hundreds of these. The CRM SDK has a list of them all in SDK/message-entity support for plug-ins.xlsx. Here are some of the most commonly used messages:
Triggered when the record is created. All set attribute values are included in the target entity.
Triggered when the record is updated. You can define which attribute updates trigger the plug-in, by defining the filtering attributes. Also, only the updated attribute values are included in the target entity (see images)
Triggered when the record is deleted.
Triggered when record is retrieved, for example when user opens a record on a form.
Triggered when a record set is retrieved using RetrieveMultiple, for example showing a view.
Next, learn about plug-in images.