A sportscaster is the person you hear talking on TV or radio while a game is happening. They tell you what is going on — who has the ball, who just scored, how much time is left — and they usually keep talking between the plays too, giving background about the players and the teams. The word is mostly American. In Britain people are more likely to say commentator. In American use it stretches a little: it can mean the person doing the live play-by-play during the game, and it can also mean the person in the studio who reads the sports news on the evening broadcast. Both are sportscasters, even though one is shouting over a crowd and the other is sitting at a desk. It is a job title, so it sounds a bit formal — in ordinary talk people are more likely to say 'the announcer' or just 'the guy calling the game'.