Tadben, the most common IPsec tunnels over public networks connecting two sites is L2L vpn, most site to site vpn connections end points involves routers or other security appliences such as PIX or ASA firewalls or VPN concetrators.
A good place to start in understanding IPsec is to read Ipsec Security architecture and framework RFC 2401 developments by IETF.
http://www.faqs.org/rfcs/rfc2401.html
link to refere to with configuration examples of VPNs, refere to Site-to-Site VPNs sections.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/prod_configuration_examples_list.html
HTH
Jorge
Jorge Rodriguez