http://wwj.cbslocal.com/2010/07/08/dangers-of-free-public-wifi/
If this article is news to you, please take the time to learn how to be secure. If the idea of doing more work to be secure scares you then please take the time to do a few minor things. Understand that when using public wifi (anywhere you don’t know who’s wi-fi network you are using) do not do anything that you would not want the person sitting next to you to see. Example: don’t do banking, online purchases, or share any personal information.
The reason you would not do these activites is you do not know who is listening or watching. It is releativley simple for a hacker to intercept (“man in the middle”) your information as you are using the wi-fi network. The interception happens because of the wi-fi you are using is available to everyone around you.
If you do intend to use public wifi please use a VPN or some other method to secure your information.