Most commonly provided by 802.11b, which uses the 2.4GHz microwave band, designated for low-power unlicensed use by the FCC in the USA in 1985 (Reserved because 2.4GHz is the resonant frequency of water, and that's how a microwave oven works -- 2.4GHz waves break bonds in water!).
There are two primary modes an 802.11b wlan can operate in: AdHoc (or PeerToPeer) mode, or infrastructure mode. Infrastructure mode typically offers better security than AdHoc mode. 802.11b AdHoc networking requires 2 or more computers, each with a wireless interface of some sort - PCMCIA, PCI, ISA or USB. Infrastructure mode also requires a BaseStation, such as an AccessPoint.
You may also want to read about FresnelZone, a phenomenon which occurs with long range(?) wireless links.
CRCnet is a project run by the WandGroup that is setting up WirelessNetworks between WaikatoUniversity and some remote schools. They gave a presentation to WLUG about WirelessNetworking -- see MeetingTopics.2003-06-23.