An AccessPoint ("AP") is a physical device that allows wireless users to access a wired network. Thus, APs are designed to act as the wireless equivalent of an ethernet hub or switch. They allow multiple wireless clients (NICs) to connect to one central hub in InfrastructureMode (aka. BSS) 1. From the physical connectivity point of view (think twisted pair or Cat5 wires of traditional ethernet), they act as a StarNetwork. Each wireless client talks to every other via the AccessPOINT.
"Basic Service Set" Mode (1)