The nozzle with a bulb is normally closed and is used in systems with conventional activation. In the top of the nozzle there is a glass bulb that keeps a piston in place. The piston prevents the nozzle from leaking.
The open nozzle is normally open and is used in systems with intelligent activation. The water will pass through the nozzle creating small droplets as soon as the pipe to the nozzle is pressurized with water.