As mentioned, blocking is done usually by the middle blocker, and depending on the side the offence is spiking from, the power or setting will help block. Blocking consists of jumping as high as the player can, arms raised, fingers spread, and penetrating the net in hopes of getting the ball to hit one's hands so that the ball will reflect off their hands, and back into the offensive court.
Here is a photo taken off flickr.com of players attemping to block the ball.
Spiking, or attacking, can be done by almost anyone on the court. The power player is the one who most commonly will spike a ball. However, middles and offside hit the ball too. There can also be a back row attack, which means someone in the back row can hit a ball, as long as they don't cross the 3 meter line. A player has an "approach" when getting ready to attack. It includes three steps:one large step, a shorter step for momentum, and then the jump off. Arms also have a specific routine when hitting.
There are a few types of spikes.
Rainbow/High ball
These are when the setter sets the ball high for the power player to attack. The most common type of attack.
Shoot
These are when the setter sets the ball fast and not very high to the power position. These are quick hits and make it difficult for a blocker to get infront of the ball in time.
Middle Quick
The setter will set the ball to the middle player, who is already approaching the ball to attack when the ball is in the air. They are usually just a foot over the net and are very powerful. The ball will usually go straight down.
Back Row
Although its already been mentioned slightly, the back row attack is often strategically planned, or if the setter can get to the ball and pass it nicely to the front row, the setter will set the ball to someone in the back row who will attack the ball similar to if it was front row. The only difference is the player can't jump after the 3 meter line, but they can jump before it and have their feet land after it.
Here is a photo taken off flickr.com of a front row attack.