Since the inception of wireless networking there have been two types of wireless networks: the infrastructure network, including some local area networks (LANs), and the ad hoc network. Ad hoc is Latin meaning "for this purpose." Ad hoc networks therefore refer to networks created for a particular purpose. They are often created on-the-fly and for one-time or temporary use. Often, these networks are comprised of a group of workstations or other wireless devices which communicate directly with each other to exchange information. Think of these connections as spontaneous networks, available to whomever is in a given area.
An ad hoc network is one where there are no access points passing information between participants. Infrastructure networks pass information through a central information hub which can be a hardware device or software on a computer. Office networks, for example, generally use a server to which company workstations connect to receive their information. Ad hoc networks, on the other hand, do not go through a central information hub.
These networks are generally closed in that they do not connect to the Internet and are typically created between participants. But, if one of the participants has a connection to a public or private network, this connection can be shared among other members of the network. This will allow other users on the spontaneous ad hoc network to connect to the Internet as well.
Ad hoc networks are common for portable video game systems like the Sony PSP or the Nintendo DS because they allow players to link to each other to play video games wirelessly. Some retail stores even create networks within them to allow customers to obtain new game demos via the store's own ad hoc network.
An ad hoc network can be thought of as a peer-to-peer network for the wireless age. Peer-to-peer or workgroup style networks were used to create a network environment for early Windows computers. This allowed these early computers to connect to each other to exchange information, usually in a smaller office environment without the need for domains and the additional management and overhead that comes with them.