That's a good question, and I think this is something everyone could chime in on. For all intents and purposes, a light is a light. But that's where the similarity ends. And because there are so many choices these days, there are many people who amass collections while having one or several goto lights. That's when lights become very use-specific.
Something to keep in mind: The BEST light is ALWAYS the one you have with you when you need it; much like a knife...or anything else. Here's some things to look for.
INCANDESCENT vs LED. Incandescent is the old filament style. MagLight is a popular example. Newer lights are using LED mostly.
FLOOD vs FOCUSED BEAM. It's a preference. Where focused beam has been the rage, flood (or wide angle) flashlights are gaining popularity. Depending upon strength, a flood would be great to light up a room during a power outage.
As a longtime backpacker and outdoorsman, I've always found focused beams with little to no spill lacking (try looking through a couple paper towel rolls and you get the picture). While it's not really necessary when in the outdoors, because you don't need a lot of light, if any at all, I've always wanted something more; something that lights up the surrounding area and trail I'm on.
Of course focused beams are fun and have their place, especially if it's a light used in the work environment, military, or police personnel. They're great for pointing out and isolating wildlife, blinding a potential threat, or highlighting a feature (much like a laser)...or in the case of a high intensity focused beam - for use as a searchlight. They're also fun in a spooky-campfire-story sort of way, because you have to turn and point the light in the direction of a noise, intruder, etc...instead of just looking.
Once again, it's all preference. And it all relates to you and what you like.
And you're right, most websites don't disclose these things. But they do categorize in terms of tactical, edc/every day carry, search lights, bicycle lights, headlamps, keychain lights, work lights/lanterns, uv lights, intrinsically safe, and so forth. There's also something called go big or go home. It's the overkill light, the fun light, and it's used mostly for showing off to your friends. Hey, it's another toy have fun with it!
The second best way to find out what your preferences are is to read up on reviews and ask questions in forums. Figure out what you're looking for specifically, and then talk to people, even call internet companies and ask questions. The first and very best way to discover your preferences and learn about lights is from personal experience. You can't know what a flashlight is capable of until you get it into your hands in real world situations. You're going to get one sooner or later, and it probably won't be your one and only final purchase.
I can't post urls here, so do some googling and read reviews. And by all means search the forums and ask questions.
That's just some of what I know from my own experience. Maybe some other more seasoned members can chime in? I just joined today.