In sports broadcasting, a commentator is the person who is saying what is happening in the game.  In the case of television commentary, the commentator is usually only heard and not often seen.   In North American English,  a commentator is also called an announcer or sportscaster.  Often, the main commentator (called a play-by-play in North America) works with a color commentator and sometimes a sideline reporter. With big events, many sideline reporters are used.

Types of sports broadcasters
Play-by-play announcers are the primary speakers. It is important for them to be easy to understand. They also have a good ability to describe the what is happening quickly during fast-moving sport. Play-by-play announcers are more likely to be a professional broadcast journalist. The name comes from the fact that they describe each play of the game as it happens.
Color commentators bring experience and insight into the game. They are often asked questions by the play-by-play announcer to give them a topic for talk about.  Color commentators were often players or coaches in the sport being talked about. The name from the fact that they provide more color to the broadcast. They provide extra interesting information that makes the broadcast more entertaining.
Sideline reporters are reporters who are put in different spots around the venue. They give the color commentator more information to give to the audience. For example, some sideline reporters could be in the dressing room area while others could be at the team benches. The often interview players and coaches before, during and after the game. They also try to get more information on things such as injuries to players.

Most sports television broadcast have one play-by-play announcer and one color commentator.  An example is NBC Sunday Night Football in the United States. It is called by Cris Collinsworth, a former American football receiver, and Al Michaels, a professional announcer. In the United Kingdom,  there is not as much of a difference between play-by-play and color commentary. Two-man commentary teams usually have a person formal journalistic training but little or no sports experience leading the commentary, and an expert former (or current) competitor dealing with analysis. There are exceptions to this. All of the United Kingdom's major cricket and snooker commentators are former professionals in their sports. The well known Formula One racing commentator Murray Walker had no formal journalistic training and only a small amount of racing experience of his own.

Using a play-by-play announcer and one or more color commentators is standard . In the past it was much more common for a play-by-play announcer to work alone.

United States
Although sports broadcasts started in 1912, the first sports commentary was broadcast in April 1921. It was done by Florent Gibson of the Pittsburgh Post newspaper. He was covering the fight between Johnny Ray and Johnny "Hutch" Dundee at the Motor Square Garden, Pittsburgh.

In the United States, nearly all professional sports teams, most collegiate teams have their own Sports commentators. These people are usually thought of as the voice of the team on radio broadcasts. They are often considered a part of the team like the players or the coaches. Television networks and cable channels will also have their own group of play-by-play announcers that work with different teams.

References

Other websites 
 Sportcaster Chronicles  - Internet radio show in which John Lewis interviews leading American sports announcers.