18650 Lithium-Ion batteries are the workhorse of modern consumer (and not only consumer) technology. If you have anything that is battery operated and there is a "Li-Ion" mark anywhere on it, probably it is powered by 18650 Li-Ion battery. Laptop batteries, cordless drills, cordless tools, e-cigarettes. All of them (probably) have 18650 cells inside.
In this video, I will just go with the basics of 18650 cells. Dimensions, protected / not protected difference, weight and so on. Expect more in the future!