An icing syringe is similar in principle to an icing bag (aka pastry bag), though instead of being a bag that you squeeze, it is a tube with a plunger.
You attach the desired decorative nozzle tip at the nozzled end of the syringe, load icing in through the top of the tube, insert the plunger, and press. You hold the tube with one hand, and press down on the plunger with the other.
Some are made now like guns, with a trigger handle that you squeeze. They are easier to use with one hand.
Most are made of solid metal or plastic, but some now have see-through plastic on the sides which allows you see to how much icing is left (though you can usually tell by how far the plunger is in.)
It is easier to swap nozzles on an icing syringe than it is on an icing bag.
The nozzles are made of plastic or metal.
You can use icing syringes to decorate items, or to pipe small amounts of filling into things.