Jet: A component controlling flow of a fluid, restricting by means of a calibrated orifice size. In the case of a carburettor this fluid could be fuel or air.
Float Valve: A component designed to allow or prevent the flow of fuel. A float valve prevents fuel flow when the level in your float chamber is correct. Exactly like your toilet cistern.
Metering Screw: A tapered screw allowing adjustable flow restriction of fluid. (In this case fuel and/or air.)