Polo is a team sport played on horseback where a mallet is used to move a ball to a goal for scoring points.
There are two main variations: Outdoor and Arena (Indoor) The basic differences are the field size, number of players and the ball.
Outdoor has 4 players per team and uses a hard plastic ball about the size of a baseball. The Outdoor field is grass and is 300 by 160+ yards, nearly 6 football fields.
Indoor has 3 players per team and uses an inflated ball which is just like a mini-soccer ball. The indoor field is typically dirt and only 100 yards by 50 yards, slightly smaller than a football field. An indoor (arena) field is typically boarded (barrier to horse and ball) and rarely has a roof.