It is a single use item and one unit takes up one weapons slot in the inventory. It's a very powerful weapon, and should be used with caution. Always charge the weapon before throwing it, and aim up.
They are usable only in the missions Great Deal (where they can be found in a box behind one of the cars after the deal goes wrong) and You Lucky Bastard (where they can be found in another box on the other of the train in the harbor). They are useful for clearing out the barricade on the first floor of the garage in Great Deal. It is later seen in Bon Appétit, but appears only in a cutscene. It is also used against you in The Death of Art about half way through the mission.
You can find more real world information about the grenade on its Wikipedia page.