uSync is actually made up of a number of projects, each one doing its own element of the wider syncing process.
Core does all the serialization/de-serialization of elements to and from Umbraco.
Within the Core all the main mapping, and comparisons are performed - making sure that items exported from Umbraco can go into another installation and have things like links and content node pointers maintained.
BackOffice is what most people think of as uSync. the BackOffice element handles the files on the disk, starts the imports and exports, passing each file to the Core
BackOffice also manages the save and delete events - to capture changes as they are made inside Umbraco.
ContentEdition is the addition of two new Mappers to uSync.BackOffice. One for Content and one for Media.
Content Edition also imports uSync.ContentMappers which has additional Content Mappers that help with the translation of internal IDs between Umbraco sites.
Snapshots is a slightly different way of working with uSync, instead of having uSync save all the changes all the time. Snapshots lets you create single point in time changesets of what is happing inside Umbraco.
uSync.Chauffeur is a set of commands that can be added to Chauffeur, so you can run uSync commands from the command line.