SQL Injections and Cross Site Scripting.

Two of the most common and dangerous attack vectors used on websites to extract information and gain access to servers are SQL injections and Cross Site Scripting(XSS). Anyone coding web apps should be aware of these security issues.

More more info - check out

SQL INJECTIONS:
Podcast: Security Now ep 87 // you can skip the first 15 or so minutes of this.
Java Examples // how to :P

XSS:
Podcast: Security Now ep 85 // part 1 - injection and xss
Podcast: Security Now ep 86 // part 2 - xss
More java fun // how to