When running DirSync:
New user, group, and contact objects in onpremises ActiveDirectory are added to Office365; however, Office365 licenses are not automatically assigned to these objects.
Attributes of existing user, group, or contact objects that are modified in onpremises ActiveDirectory are modified in Office 365; however, not all onpremises ActiveDirectory attributes are synchronized to Office365.
Existing user, group, and contact objects that are deleted from onpremises ActiveDirectory are deleted from Office365.
Existing user objects that are disabled onpremises are disabled in Office365; however, licenses are not automatically unassigned.