The Garden Tools advertisements might suggest that this is an easy, non-smelly, practically instant way to turn trash into great garden soil.
A compost tumbler is usually made from a barrel-shaped plastic container that's been placed on a stand. The tumbler is turned by the gardener.
The turning action incorporates air into the composting leaves and grass clippings, and mixes the various materials together.
The regular addition of air keeps the microbes working at their maximum potential.
Everything that goes into the composter needs to be chopped into the smallest bits you can manage, and the wet and dry, (or green and brown) materials need to be in the correct proportions in order to give the composting bacteria an optimal environment for their work.
Turning the barrel replaces the work of turning a compost pile with a garden fork, which is definitely the hardest part of making "hot" compost.
A number of companies make these devices, and some manufacturers even suggest that it's such a "nice" way of making compost that you can put your compost tumbler on the back porch.
The tumblers appear to have been invented for urban and suburban gardeners who don't like the looks or potential smell of an old-fashioned compost pile.
The advertisements do make the process look quite Garden Shredder sanitary.