IPsec Site-to-Site VPN
Site-to-Site VPN generally refers to two organisations that create a VPN tunnel connection between them to be able to securely share data over the internet. The two organisations would require IPsec VPN devices and an internet connection to be able to establish a VPN tunnel.
The VPN tunnel does not necessarily have to be over the internet. It is common to establish VPN tunnels on other transmission mediums, even within a local area network itself or a private leased line.
IPsec VPN tunnels support a number of different topologies depending on the requirements, which is covered within the VPN Topologies page.
Further Reading
Wikipedia's guide to VPN