The document defines key network infrastructure concepts including the Internet, intranets, extranets, virtual private networks, and security devices. The Internet connects worldwide computer networks using TCP/IP and contains resources like the World Wide Web and email. An intranet is a private network within an organization, while an extranet extends access to partners. A VPN uses encryption to create a secure "tunnel" between devices on different networks. Firewalls, proxies, and other security devices help protect networks from threats and control access.