A VPN is a safe link set between two end-points over an IP network : Both routers authenticate, data are encrypted and each device of a LAN can exchange data with each device of the other one.
Two types of VPN can be set : OpenVPN VPN and IPSec VPN.
IPSec has the advantage to be a standard solution.
OpenVPN is easier to employ because the transport layer is TCP or UDP; that is why, it can be easily used when the VPN must pass through several or even numerous company routers.