An ad-hoc network is a wireless network where each device (or node) is capable of sending data to every other node on the network directly, instead of moving all data through a centralized device like a router. Ad-hoc networks are sometimes used to create wireless "hotspots", where a single internet connection is shared among many devices; most computers are capable of sharing internet using ad-hoc wireless as well. Wikipedia has a general overview of ad-hoc networks, as well as many different routing protocols used to route data across ad-hoc networks.
Questions possessing this tag are generally relate to creating and connecting to ad-hoc networks on computers. Mobile devices are also capable of creating ad-hoc networks; questions about ad-hoc on Android and iOS are welcome on their respective sites.