If you, as the end user, initiate a file transfer with a remote computer and no one on the other end has to do anything (automated response) that remote computer is considered a server. If you run a program that makes automated responses to requests then you are running a server.

An example of a transfer that is not considered serving is if you initiate a file transfer (AIM file transfer or similar) from you to a friend. This request requires both ends to do something in order for the transfer to take place. After the transfer is complete the program is closed and is not dedicated. This is a very simplistic view, but accurate.

VPNs and remote desktop access applications are considered acceptable.