They are a different form format of Lithium Ion chemistry. So they have high energy density relative to older cells like Alkline, NiMh, NiCD. They also have a higher C-rating. That wont affect anything like a flashlight but would on any device that delivers high current relative to its nominal storage capacity (power tools). You need a charger that 'knows' this chemistry because it has a specific input voltage & ramp-up cycle. Which brands I cant recommend because they vary so much & who knows what's under the shrink wrap anyways. 90% comes from China but a myriad of grades. A lithium no-no is to allow it to drop below its terminal voltage. If it goes dead, its done. Also be aware of some Lion smart cells (or whatever they are called). I don't think they are as popular in this form factor but basically they have little on-board circuitry in the cell end that helps manages the charging & discharge duties.
Back to your basic question, I have an Amazon LED flashlight that I can either use an 1860 or 3 AAA cells, just swap out the plastic adapter. The math says Amazon Basics like alkaline cells were actually better $/mah value so that's what I run.