Also known a a "hat box" due to its shape, a sharps container is used to contain biohazards such as used needles (they're sharp, hence the name "sharps" container). Usually they have a bit of a rubber flap over the top through which you can push things, but built so that it's hard for anything to fall out.

Here in Australia, the sharps containers are a bright yellow with a cute red biohazard warning sign on it. Neat black lettering saying that it is a sharps container and that it is only to be emptied by authorized personnel.

Built of thick, rigid plastic so it's highly unlikely that anyone would get a needlestick injury just by carrying it around.