The Airport Express can either act like a mini-version of an Airport base station (IE, as a wireless access point or full-fledged router, by dedicating its Ethernet port for network uplink) *or* it can operate like a bridge, connecting as a client to an existing wireless network and allowing you to then connect wired computers directly to it or via a hub.

One thing: I could be mistaken about this, but I was under the impression you had to be using another Apple Airport to host your wireless network for the bridge mode to work. (IE, if you're using any other brand of WAP you're SoL.) Is that true or not? I seem to recall it was at the time I debated buying one to solve a bridging problem. (And thus didn't buy it.)