A Wide Area Network is as its name suggests a network that connects devices over long distances. This is in contrast to a LAN (local area network) which is used to connect devices that reside locally. LANs are built on distributed networks such as Ethernet with each device connecting through a local switch or wireless access-point. WANs on the other hand tend to be point-to-point connections between to routers.
WAN links traditionally were point-to-point links that spanned regions, continents and international destinations. They were typically expensive, in relation to the bandwidth offered, and service was reliant on the quality of the service provider networks at both ends of the link. Troubleshooting international links could be awkward as two or more service providers could be involved.
The common WAN technologies are built upon service provider link such as Serial, ATM, ADSL, T1 & E1 private lines, and Frame Relay. However, virtual circuits that utilize the internet and service provider core IP networks have superseded most of these core technologies. These virtual circuits operate over IP/MPLS and can provide the same services as private wire WAN links but at a fraction of the cost to both service provider and end customer.
WAN links nowadays tends to be built upon ADSL and virtual circuits, both virtual private wire (which simulates a leased line) or through MPLS VPNs. By using virtual circuits to build the WAN links is beneficial to both the service provider and the customer as they can provide full meshed networks were previously only hub and spoke connectivity would have been financially feasible. A hub and spoke network topology is where a central site has links to each remote site but remote sites have no connections with each other, except through the hub site. All traffic therefore must be direct through the hub site and this can be very inefficient. A full meshed network has each site connect to all other sites and this is very efficient but in traditional fixed line networks hugely expensive.