A block in basketball is a successful deflection of an attempted shot by a defender. Defenders often play with their hands up or out, ready to stop any pass or shot attempts by an offensive player. Blocks can be made anywhere on the court, but are most commonly seen at the low post, close to the basket. That is due to the positioning of defenses, where taller players stand near the basket. Tall players have an easier time blocking shots than say a point guard, because of their long arms and large frame.
Blocks are somewhat of a risky move by a defender. While they do have the chance of potentially erasing points from the other team, they can also result in a negative outcome for the blocker's team. Two of those negative outcomes are shooting fouls and goaltending.
There is a chance that the shot blocker may make contact with the shooter while attempting to block the shot. That results in a foul, and free throws for the offense. On top of that, the block can be called a goaltend, which results in the shot counting. That occurs if the ball is already on its way down in terms of its arc. With that being said, sometimes contesting a shot can be better than blocking it.
One thing that most shot blockers have in common is their height. Most of them are tall people, commonly with broad shoulders, and long arms. That combination allows them to reach further and get to balls easier.
Block is also a term used to describe an area on the basketball court. It is referred to as "the block" and is located along the free throw lane close to the basket. It serves no definitive purpose other than providing a mark on the court to help players with their positioning. Most players post up in the block, and the square marking makes it easier to do so.