A Penlight is a small, pen -sized flashlight , often containing two AA batteries or AAA batteries . Penlights can be made out of plastic or metal, and can be any color . They most often come with a clip, to hold it on your pocket or something. 
A penlight usually uses a small light known as an "LED" (Light Emitting Diode) but it can also use a small lightbulb made just for small flashlights. The LED lights up when power is run through it.
Making the penlight light up is as simple as pressing a button, or screwing the end on tighter so it makes contact with the batteries. LED penlights are becoming increasingly common with both regular and COB LEDs.  AAAA are a newer, thinner type of battery, most commonly found in small electronics .
Low-cost penlights may be disposable with little or no provision to replace batteries or bulbs, and are sometimes imprinted with brands for advertising for promotional purposes.