What are the criteria that differentiate a chess grandmaster from a
chess international master?
The current FIDE rules regarding the awarding of international titles are described in FIDE Title Regulations effective from 1 July 2017. Basically there are two ways to achieve a title - via norms or directly via achieving a given result in a world or continental championship or in an Olympiad.
The level of achievement required to be awarded a direct title are described in Table for Direct Titles effective from 1 July 2017.
For instance the GM title is awarded to the top 16 players in the World Cup, the winner of the Women's World Championship, the outright winner of the World S65, S50 and U20 Championships, the outright winner of Continental Championships and the Continental S65, S50 and U20 Championships.
The IM title is awarded to the losing finalist in the Women's World Championship, the silver and bronze winners in the World S65, S50 and U20 Championships, the winners of the World U18 and U16 Championships, the silver and bronze winners in the Continental Championships, the winners of the Continental S65, S50 and U20 Championships, the winners of the Continental U18 Championships.
Qualification via norms requires 3 (or 2 for very long tournaments) "norm" level performances in high level tournaments combined with reaching a certain rating. These rules are complicated and you should refer to sections 1.4 and 1.5 of the document on the the FIDE website.
