Synchronization is the process of moving cached files at replica locations back to the target (master) vault. By default, it happens every 3 hours, but you can change that setting of course. In our case, we have File Servers (replicas) in multiple regions, and every 3 hours, the files that are uploaded to those servers come back to the master vault.
I believe the two types of replication are called ad-hoc and scheduled (could be wrong on the scheduled part). Ad hoc replication copies a file to the user's location (replica) at the time they request the file if it doesn't already live at their location. The first time they access the file, it is slow, but every time they access the file after, it is fast since it is now local to them. Scheduled replication can be setup to replicate objects to locations where you know people will be using them heavily. For example, you could replicate an entire product to a replica and then when a user accesses a file from that product, it is already local to them, so they don't have the initial "download" time they would have with ad-hoc replication.
That's a pretty high level overview, but hope it helps.
~Jamie
