The very first one is an aptitude test
its pattern is as follows
u will get 36 questions on progressicve matrices.....in which 8 figures will b given and u will have to predict the 9th figure...based on pattern matchiing.....the book for this is " progressive matrices......by JC RAVEN" or "VERBAL AND NON-VERBAL REASONING by R.S.Agrawal", it would be good if you opt the former one since the latter may not contain the exact questions.
next comes the GD(was not exactly a GD for me, they asked every one to give a talk on any subject they liked)
next the tech interview, it depends on the panel, some panels are completely concerned with techincal questions, others a mixture of tech and HR, even some of the people didnt get any tech questions. It would be good if you concentrate on any 3 core subjects, eg:- OS,C++,C,DBMS etc
next the HR, i think there wont be much problem in HR, I was asked to explain the topic i took for GD.
note that each level is an elimination round