IPsec site-to-site tunnel
IPsec tunneling theory
IPsec is very powerful, but also quite complicated and intricate. When starting on tunneling, you might want to start exploring the site ipsec-howto. It explains both the theory of IPsec, and describes how to create IPsec tunnels. However, after reviewing the theory, you might want to continue here for a more elaborate, Debian-specific, site-to-site only, howto on tunneling.