Web applications on uncompromised computers are vulnerable to XSS,CRSF,sql injection attacks and cookie stealing in unsecure wifi environments.
To prevent those security
It definitely is not enough! There are several other security issues you have to keep in mind when developing a web-app. To get an overview you can use the OWASP Top-Ten
I think this is an very interesting post to read when thinking about web-security: What should a developer know before building a public web site? There is a section about security that contains good links for most of the threats you are facing when developing web-apps.
The most important thing to keep in mind when thinking about security is: Never trust user input!
[I am answering to this "old" question because I think it is always an actual topic.]