GDs can be topic-based or case-based.
Topic based GDs can be classified into three types :-
1. Factual Topics
2. Controversial Topics
3. Abstract Topics
Factual topics are about practical things, which an ordinary person is aware of in his day-to-day life. Typically these are about socio-economic topics. These can be current, i.e. they may have been in the news lately, or could be unbound by time. A factual topic for discussion gives a candidate a chance to prove that he is aware of and sensitive to his environment.
E.g. Democracy is a luxury in India, Indian Railways, State of the aged in the nation.
Controversial topics are the ones that are argumentative in nature. They are meant to generate controversy. In GDs where these topics are given for discussion, the noise level is usually high, there may be tempers flying. The idea behind giving a topic like this is to see how much maturity the candidate is displaying by keeping his temper in check, by rationally and logically arguing his point of view without getting personal and emotional.
E.g. Reservations should be removed, GDs as a means of selection by the IIMs are a farce, Women make better managers, Relegion & Politics.
Abstract topics are about intangible things. These topics are not given often for discussion, but their possibility cannot be ruled out. These topics test your lateral thinking and creativity.
E.g. When red meets red, Twinkle twinkle little star, Virtue has its own reward, The number 1.
Another variation is the use of a case instead of a topic.
The case study tries to simulate a real-life situation. Information about the situation will be given to you and you would be asked as a group to resolve the situation. In the case study there are no incorrect answers or perfect solutions. The objective in the case study is to get you to think about the situation from various angles.
IIM A, IIM Indore and IIT SOM Mumbai have a case-based discussion rather than topic-based discussion in their selection procedures.