How does OS X connect to other computers on the LAN? I am behind a router, with some windows users, and I noticed that their computers show up in my finder, under shared. I never closely examined this feature, but today I decided to connect as my user on the windows computers. What I noticed is a directory labeled Users popped up. Clicking on this, and then my user directory, I see the normal OS X user folder structure, but the files are the ones on the computer.

This looks like FTP to me, but the windows computer has no special configuration, so I don't think that that's the case.

How does the OS X connect to other computers on the LAN, when they have no special configuration to do so? What service is this?



OS X 10.11 El-Capitan
There are several Protocols in a LAN Environment which a Mac can use (AFP, SMB etc.). You can connect to a PC's usually (or a PC Share) via SMB. 

In order to connect to that PC you have to provide credentials, a Username and a Password. If you have a local Account (ignoring complex Network-, Server- and Domain-Architecture for a moment) on that Computer you can use these Credentials and have access to all Folders for that User-Account, including a 'Documents' and 'Desktop' Folder. So you are not actually seen the Folders from your OS X User-account, but the default Folders from the Microsoft OS which happen to be Named the same ('Desktop' etc.). There are differences, f.e. on OS X you have a 'Library' Folder which you don't have on Windows etc.

FTP is a completely different Protocol, requiring an FTP Server and Credentials. I doubt that this is the case in your environment, even though you can access it via Finder (on the menu bar, click Go > Connect to Server and enter "ftp://test.domain", if "test.domain" is the name of the Server).