Bridging two networks
Normally, when you think about bridging two networks, you think about connecting them through a wired bridge, such as when you hook up an AP to share your broadband Internet connection via a router.
Actual wireless-to-wireless bridging is a new thing, made possible by the new Wireless Distribution System (WDS) standard. WDS allows traffic to flow from one AP to another as if it were traveling from Ethernet port to Ethernet port on a wired network. Best of all, your AP can function like an AP at the same time. It doesn't have to be locked into a dedicated role, like an AP used as a router.
You also could use two APs using WDS to extend coverage between two buildings by attaching a directional antenna to each AP or allowing for line-of-sight through a window over short distances. If you use APs combined with directional antennas, they won't be able to serve clients unless the clients are in the path of the directional signal. Still, with the price of access points dropping, this could be an easy way to connect two WLAN segments over a greater distance and with better performance than other options.