Different Implementation of VPN
I just want to ask gurus out there what are the different implementations of VPN. I just learned that there is a technology called IP VPN which connects branches from the ISPs. I mean is it true that this method, both branches doesn't need to be connected to the internet and to build the tunnel? I know one VPN implementation which is if both branches do have internet connection, I can build a tunnel so that both ends would meet in the internet. They said that IP VPN is a different approach and is faster than the method that I know since both are automatically connected with each other even without authentication and everything. Now, how secure is this approach if they no longer need authentication? Or do they dial from the central branch to connect?