Some wireless networks, such as those offered by hotels and hotspots, are not encrypted (they don't require a pass phrase to connect) but have an authentication web page. For example, you connect to an unsecured airport network, and it asks you to login using the credentials you were given when you paid with your credit card.
How secure are these type of networks? I would imagine that, since the connection between the client and router is not encrypted, and since authentication does not mean encryption, it would be entirely possible to sniff the data exchange. But if it's unsecured, why do so many universities, hotels, and airport hotspots use this system?