18650 batteries are used for powering everything from laptop batteries to electric vehicles. It is a standardized type of lithium-ion battery, cylindrical in shape and measuring 18mm in diameter by 65mm in length (give or take a few 1/10s of a millimeter).
They are often used alone or in groups to build battery packs and power banks, which are then used to power or recharge a device. Although most of the time they aren’t easily accessible to users, there are a number of devices that use 18650 batteries directly, and in a way they can be removed and replaced.
Many of these items happen to be flashlights but, increasingly, a number of other kinds of 18650 devices have become available.