Replication clarity?

Robbo777

Regarding replication and just how it actually functions and what is replicated also, i'd just like a bit of clarity on a few questions i have.
One being the RPC and DFS protocols and services, i think i am right in saying that RPC is used to "call" other domain controllers and ask them for certain services in getting replication to work. I have heard that DFS is used in this regard also somewhere, does RPC "call" on the traget domain controller to use the DFS service and then copy and sync over all the data that needs to be replicated. Or am i wrong in saying that?

Also, what is actually replicated? Is it files, active directory groups, users etc... all of those objects and the DNS SRV records are left to DNS replication?

Thanks for help