They are all like thick aluminum foil that can be unfolded into a cylinder that encloses the whole burner part of the stove. When there is no wind, they still are great for focusing the heat on the pot more so and diminish boiling time. Finally, a word about cleaning needles. All stoves have a single jet that all the gas vapor blows out from. Sometimes this jet gets clogged with carbon or something and a fine wire, called the cleaning needle must be inserted to poke the hole clear again.
Most stoves come with such a needle as a separate tool.
Some have built in needles. The MSR XGK has one that is employed by shaking the stove. The ancient Optimus' mentioned above had one deployed by simply turning he valve to one extreme end of it's adjustment range. Below, some of the main players are described. This stove, uses the MSR idea of employing the fuel bottle as the fuel tank.