This comes from a slightly related question about drag from landing gear.
A tail-dragger is the old-school layout of aircraft, particularly WW2-era fighters. It's also known as Conventional Landing Gear.
The first two landing gear is usually retractable, but the tail wheel is not.
At least, I have never seen such a configuration with a retractable tail wheel. Has there ever been one?
Note: question excludes now more common tri-cycles.