These LED flashlights are powered by anywhere from one to two AAA batteries. The advantages of AAA flashlights is the common and inexpensive battery type. In addition, the thinnest form factor makes for ideal "pen" light or keychain flashlight.
The disadvantage is that AAA batteries are only 1.5 volts, so these lights aren't typically as bright as their 3 volt lithium CR123A brothers. They also don't last as long and can't boast about having a 10 year shelf life, like lithiums can. Since this is the smallest form factor, they also have the least amont of power and capacity.