the work the entire deltoid muscle with a focus on the front and side. This movement also works the triceps, but pretty much any pressing movement will work the triceps.
This exercise can be performed seated or standing, but I would suggest having a spotter to help you get the weight into place. Start with the weight extended over your head, to start the movement brings the bar down slowly behind your neck. Bring the bar to approximately half way down your head, but if you feel too much stretch in your shoulders do not go as far. Next, press the bar back overhead to the starting position. This exercise will take some getting used to for your body, but once you find the groove you should be able to do the same weight that you use for a standard military press.
This movement is not for everyone, people that lack flexibility may feel pain in their shoulder. Work into this movement slowly, make sure you have properly warmed up before starting this exercise.
You can also do this exercise as a push press movement with the bar started behind your head. Begin with the bar on your back like a squat, and then drive with your legs and press the bar overhead.