Cross Site Scripting (XSS) is a code injection vulnerability found in web applications and is generally used by malicious hackers to hijack a legitimate user's session with the website. XSS vulnerabilities are caused because of improper validation of user input by the Server and then sending this invalidated input back to the user in some exploitable form. A great resource to track the latest XSS vulnerable software, websites and latest research is XSSed.com
1. Quick Overview: This video explains the basics of XSS, kinds of XSS - Persistent, Non-Persistent and DOM based.
2. Protecting your Website against XSS Attacks: This video explains various techniques which can be used to mitigate XSS vulnerabilities on your website - input / output validation, modification of output tags etc.