You can use the Express without a Basestation but you have to have *something* connected to the internet.
If you want to use the Express as a wireless hub in place of a Basestation, you can, but it has to be connected to a cable/dsl modem or a network with internet. It will allow up to ten users with 802.11b/g capability to enjoy wireless access.
If you have a Basestation network already, you can put the express anywhere you want (next to your stereo, or where you want your printer) and it will extend the range of your network, as well as giving you the wireless printing and Airtunes access.