VPN explanation

Hi,
I could do with a little help understanding logging onto a Domain over a VPN. I have been left in the lurch looking after an IT department until the new manager starts. I have limited knowledge of this level of networking. I understand how a VPN works and have set this up on the client laptop. There is an icon on the desktop which the user double clicks, authenticates and connects.(This is from the office). What I am trying to get my head round is what happens at the remote location.
I need to explain to the user how to logon to the network.