A Tinywhoop is a micro FPV quadcopter, usually 65 to 85 mm across measured motor to motor, with ducted propellers. The ducts matter: they protect the props, protect whatever you hit, and let you fly in a living room without redecorating it. The whole aircraft weighs somewhere between 20 and 60 grams and runs on tiny single-cell (1S) lithium batteries that cost a few dollars each and give you roughly three to five minutes per charge.
You will see two motor types. Brushed motors are the older, cheaper option: quiet, gentle, and effectively disposable, since they wear out after a couple dozen flight hours. Brushless motors cost more but last essentially forever and hit noticeably harder. Most whoops sold now are brushless, but not all of them: the entry BETAFPV Cetus FPV Kit uses brushed 716 motors, and its sibling, the Cetus Pro, is the brushless version. If a kit does not say brushless, assume brushed and assume you will be replacing motors.
The reason the whoop became the canonical FPV trainer is simple economics plus simple physics. Crashes are the tuition of FPV, and a whoop makes tuition nearly free: 25 grams of ducted foam-and-plastic hitting a wall at 20 mph damages nothing, while a 5-inch freestyle quad doing the same costs you props at best and an arm or a camera at worst. Because a whoop is harmless, you can fly it indoors every single day, in winter, at night, in a small apartment. Daily stick time is what actually builds the skill. And because whoops fly the same flight modes as big quads, including full manual acro, every hour on a whoop transfers directly to the 5-inch quad you will probably want later.