The things that doctors poke into a superficial vein to establish IV access.
To understand the gauge size of cannulas, just remember that the number they are are a fraction of an inch that is the diameter of the needle involved. As flow rate is proportional to the fourth power of the radius of the tube, small increases in size make a huge difference to the amount of fluid you can push through. You can easily push a litre of fluid through a 16 gauge cannula in a few minutes, whereas that would be nigh impossible with a 22 gauge.
Cannulas nowadays are usually made of plastic -- the needle itself that the cannula slides over is made of metal. Cannulas can be left in the arm/leg for up to 72 hours (some places say 48 hours while some say "until they tissue") before they must be changed because of the risk of infection.