Process does not share memory space but threads do. The context switch is more heavy in terms of processes. One process may contains multiple threads, but not the other way around. Then I was asked to explain the generation of processes and forks and so on. I took O/S in more than 10 years ago and thus I think I can't recall all the details. I just be honor with the interviewer if "I don't know" and try to provide a partial answer to my best knowledge. (They are all textbook concept, nothing fancy though)