Duck tape - Probably the original cloth backed (duck fabric) adhesive tape, much used by the military. Now a brand name of Duck Tape, which still uses the original style of tape with modern improvements.
Duct tape. Used by HVAC technicians to seal joints in ducting. No cloth backing, but flexible vinyl or plastic to effect a proper seal at varying temperatures.
Gaffer Tape - Used by gaffers/riggers etc in the entertainment/film industry. Like Duck tape, it has a cloth backing allowing durability and a lesser adhesive bond to enable quick rip up changes to cables etc on stage or film sets.
WD-40 - A water displacing fluid (WD = Water Displacing) used for removing/displacing water from vulnerable electrical components like ignition points in car distributors (remember them?). Used often by the military to get water damaged/flooded electrical equipment back up and running.
All of the above is based on my personal experience of using this stuff and finding out the differences for various applications as I've needed them.