The term is highly subjective, and any individual player might be considered a ball hog by some observers but not by others.
Ball-hogging usually involves excessive shooting of difficult shots, especially when other players are in advantageous positions.
They also tend to have a very poor assist-to-turnover ratio, used as the main statistical indicator of how well a player "shares" the ball.
In professional leagues, when an exceptionally competent player takes control of the ball a large amount of the time, but it is in the team's best interest due to the player's very high shot percentage and low turnover rate, and low availability of similarly skilled players on the team, he is not usually considered a ball hog.
However, in amateur and children's basketball, monopolizing the ball is often considered unsportsmanlike regardless of the effect it has on the team's wins.