CAT stands for Common Admission Test that is conducted by the six IIMs as a national level entrance test in selecting students for their business administration program at the Masters level.

As you asking for its Registration process , so here I would like to tell you its Registration process will start in septmber 2016 for your idea here I am telling you last year dates :
Publication of CAT 2015 Notification 26 Jul 2015
CAT 2015 Registration 06 Aug 2015
CAT 2015 Admit Card 25 Oct 2015
Date of exam 29 Nov 2015
CAT 2015 result declaration with score cards Jan 2016 (second week)

Common Admission Test (CAT) was held as online computer based test.
Rough sheets and pen were provided at the exam center. Candidate was not allowed to carry a physical calculator but an online calculator was given for the exam. Duration of the exam was 180 minutes (3 hours).

There were objective as well as descriptive type questions.Section Number of questions
Quantitative aptitude (QA) 34
Data interpretation and logical reasoning (DILR) 32
Verbal and reading comprehension (VRC) 34

CAT Eligibility
One must hold a Bachelor’s Degree, with at least 50% marks or equivalent CGPA (45% in case of the candidates belonging to Scheduled Caste (SC), Scheduled Tribe (ST) and Persons with Disability (PWD) / Differently Abled (DA) category).