Although switched Ethernet is now the most common data link layer protocol and IP as a network layer protocol, many different options have been used, and some continue to be popular in niche areas. Smaller LANs generally consist of a one or more switches linked to each other - often with one connected to a router, cable modem, or DSL modem for Internet access.
Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic via VLANs. Larger LANS also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, sensors and so on.
LANs may have connections with other LANs via leased lines, leased services, or by 'tunneling' across the Internet using VPN technologies. Depending on how the connections are made, secured, and the distance involved they become a Metropolitan Area Network (MAN), a Wide Area Network (WAN), or a part of the internet.
This article from : http://en.wikipedia.org/wiki/Local_area_network