A blocked shot in basketball is when a defensive player blocks/deflects the offensive player’s shot and keeps them from scoring. The thrill of an opponent’s shot being rejected and sent back makes this one of the most exciting aspects of the game.
A shot blocker’s main goal is to DISRUPT and CHANGE the player’s shot and make him MISS. If he does that, then he’s done his job. A nice highlight reel block is just a nice bonus.
Typically, the taller and larger players are known as "shot blockers," obviously due to the height advantage and playing around the basket.
Here are few important points to remember and practice that will help you become an effective shot blocker:
Time your opponent's ascension and release point; make the block right at the point of release (as soon as it leaves his hand). If you do it too early, you may miss or commit a foul. If you do it too late, you may commit a goal tending violation.
The player is moving and you're moving. Anticipate where the ball is going to BE, not where it currently is.
You're jumping into the air to block a basketball; you want to control your body so you don't go crashing into the player for a foul. FOCUS ON THE BALL.
A more experienced player will often do a pump fake (or two) to get you in the air. Be mindful of this and don't bite.
KEEP THE BALL IN PLAY
The problem with several of today's shot blockers is that they want to send a message or entertain the crowd and they block the ball as hard as they can and often send it out of bounds. Instead of doing that, keep the ball IN PLAY and block it or bat it to a teammate to start the fast break. This is something that was done by NBA great Bill Russell of the Boston Celtics.
NOTE: When you're coming around to help a teammate, getting a blocked shot is much easier since you have the element of surprise on your side. The player won't see you or won't be ready, so get in there and send it back.
Here is an excellent video from TNT featuring Charlotte Bobcat Center Emeka Okafor. He covers some of the finer points of shot blocking: