It has five power levels, from "Eco" (10 lumens - just enough glow to see your way around) to "Turbo" (850 lumens - burn out their retinas), plus strobe. You select the power level with a side button and when you click the tail cap button, it turns on at the last power level you used.
It can either use two CR123 disposable batteries or one 18650 rechargeable. I already had several 18650s and a charger, so the cost of ownership is nil.
You can easily find these lights for a good price on Amazon or eBay.