A commentator is someone who passes his opinion about any certain a topic or area of expertise to the public. There are various types of commentators ranging from topic to topic. He/ she could be a professional or an amateur at the particular topic. They are usually in sports or news organizations. Commentators are usually asked to give their opinions about a particular thing.
This helps others who are watching the TV show or news channel or match to understand what is happening at the place. They can even hear it and appreciate commentary about anything. A person who writes and/or does commentary is a commentator. It could be done radio and TV especially on news channels and sports commentaries.