Airport is Apple's implementation of the 802.11b wireless protocol. This allows ethernet-speed networking over a small (house sized) wireless LAN. Apple was the first company to implement 802.11b wireless networking to a broad consumer base.
An Airport network consists of at least one base station (which also functions as an IP router as well as a ppp modem connection if necessary) and an Airport card in each wireless computer. Some Macs (later iMacs, the Cube) can also function as the base station if so configured.