These flashlights run by anywhere from one to eight AA batteries. The advantages of AA flashlights is the common and inexpensive battery type. In addition, the thin form factor makes for a great EDC (every day carry) flashlight like a "pen" light or even a keychain flashlight.
The disadvantage is that AA 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.