Offside
There are three types of Offsides in hockey. A 'normal' offside, a 'delayed' offside and the 'deliberate' offside.

Normal Offside
Normal offside is when an offensive player crosses the blue line before the puck crosses over the blue line. This is not allowed. The whistle is blown immediately.

One linesman will skate to the position where the puck was sent across the blue line before the offensive player crossed the blueline. The puck will be dropped there. The second linesman will go and chase the puck and return to the spot where other linesman was standing. Play will resume there at that point.

Delayed Offside
An offensive player is inside the offensive zone. One of his team mates sends the puck inside the offensive zone, from outside the Blue line. Not good!

The linesman on the blue line will raise his hand, straight into the air. He is indicating to everyone that he will blow his whistle, if that offensive player does not come out of the offensive zone and touch the blue line with his skate ...before going back in to the zone. If this player comes out, play will continue. If player stays in the zone...play is stopped. Hence delayed offside.

Deliberate Offside
An offensive player in offensive zone. An offensive player from outside the blueline deliberately looks up, sees his mate inside the blue line (his offensive zone), he deliberately hits the puck back into the zone across the blueline. The whistle is blown immediately and puck is taken back to the offensive teams defensive zone. The key here is the deliberateness of the action taken!