IIT

Hello Ruthran,
The eligibility conditions are:
An applicant must have a Master's degree in engineering/technology with marks/CPI not below the specified minimum, or a Bachelor's degree in engineering or science (4 year programme) with a minimum of 75 percent marks/7.5 CPI, or Master's degree in science or an allied area, satisfying each of the following criteria:
(a) a minimum of 65 percent marks/6.5 CPI in the master's degree,
(b) first division in Bachelor's degree, and
(c) JRF/95 percentile or higher in GATE.
However, a valid GATE score is needed for the candidates with a Bachelor's degree in engineering, if they have not graduated from a CFTI (Centrally Funded Technical Institute).
Once you satisfy this eligibility criteria, you may get a call for test/interview depending upon some objective shortlisting by the admission committee. This shortlisting is purly objective, e.g. if they are too many applicants they sort them by their performance in the last degree.
After that, usually every IIT conducts a written test which is along the line of GATE. Based on this test, those whose written test scores are above a certain threshold gets an interview.
This interview usually tries to judge how likely are you to succeed in doing a good research and therefore get a degree. Apart from your fundamentals in some core undergraduate subjects, your analytical skills and your line of thinking is taken into account before deciding to make you an offer.