Anyone who has ever seen a basketball game, be it high school, college, or professional, has seen the referee blow the whistle and call a defensive foul on a player. What is a defensive foul in basketball? NBA.com simply defines it as "called after any unnecessary or illegal contact either on the ball or away from the ball." There are many different types, however, and different rules governing them.
A blocking foul is called if a defensive player intentionally moves in front of the person with the ball and attempts to impede her progress to the basket. However, if the defensive player is judged to have established position and is not actively moving, the referee may call an offensive foul instead.
Making contact with any part of the ball carrier's body while attempting to make a steal, even simply slapping the hand, wrist or forearm, is considered a defensive foul. Likewise, physically pushing, elbowing or otherwise making contact with an offensive player, whether he is in possession of the ball or not, is a defensive foul as well.
According to NBA.com, the defensive foul of goaltending is "when a defensive player interferes illegally with a shot on the rim or on a downward path to the hoop; the shot is assumed in and the offensive team receives the basket."
A defensive player can not remain in the "key" or painted area under the basket for more than three seconds at a time.
Any defensive foul committed on an offensive player who is in the act of shooting the basketball results in the offensive player being awarded free throw attempts. Free throws are also granted to the offensive team in cases where the defensive team has committed too many fouls and is over the established penalty amount (which varies by league).
If a player commits either five or six total fouls (any combination of offensive or defensive violations), she fouls out of the game, meaning that the player is ineligible to return to play during the current contest.