From next year onwards the selection process at IITs is set to change. There will be two tests 1)JEE Main & 2)JEE Advanced. Students who qualify the JEE Main will be eligible to write JEE Advanced. The other criteria being such students should be in top 20 percentile list of their respective board exam and in the 150,000 student list.
To learn more, please refer http://blog.careermitra.com/2012/06/iit-