Please explain what, why and how of kerberos authentication. I am using spring 3+. Also please elaborate on how to implement it in my java spring project.
There's an official Kerberos extension for Spring Security: http://projects.spring.io/spring-security-kerberos/
First step is to configure Spring Security for your project. Try that with for example a temporary in-memory authentication configuration, with some hardcoded users - just to check that the rest of the configuration is O.K.
Then, proceed with adding a Kerberos authentication provider, configuring the Spnego configuration and etc (everything is described in the documentation).
Check out the example (uses Java-based configuration, but it's easy to translate that to XML config): https://github.com/spring-projects/spring-security-kerberos/blob/master/spring-security-kerberos-samples/
And the documentation: http://docs.spring.io/spring-security-kerberos/docs/1.0.0.RC2/reference/htmlsingle/