Tomcat authentication setting

I do remember in tomcat it requires to add role and user credential in tomcat-users.xml file for adding security to web pages. Sometime i need to refer some sample. Here is the sample's..

http://www.indicthreads.com/1443/setting-up-secure-web-authentication-in-tomcat/
http://www.thecoderscorner.com/tcc/a/web-tomcat/role-based-security-and-authentication-with-tomca

http://tomcat.apache.org/tomcat-6.0-doc/realm-howto.html#What_is_a_Realm?