The Basics

LAN: Local Area Network: Geographically small collection of machines connected by a network. Another common definition says that a LAN is a network of machines separated by other portions of the network by a router.
Protocol: Formal specification of the rules of a network service or communication method
TCP/IP: Transmission Control Protocol/Internet Protocol: The most popular set of rules by which machines communicate across a network
Firewall: Hardware and/or software at the entrance to a network that controls what kind of traffic can be shared between hosts inside the network and the Internet.