uSync8 has several extension points that allow you to control how data is imported and exported and listen for events so you can, for example, do things after an import is performed.
When content is serialized or deserialized the property values can be manipulated via value mappers. These mappers allow you to identify any dependencies required for a property or map internal ids to more generic values for export (in practice for Umbraco v8 you should always store UDI or GUID Values and then the mapping is not required.)
Handlers manage events and disk IO - they take serialized items and save and read them from the disk they also handle save, delete and move events.
A Serializer handles the streaming of the Umbraco Content to and from the Database to XML (XElements).