I think Marvel Studios backed into the Infinity Stones (nee Gems) as their Cinematic Universe MacGuffin*, which is part of the reason why they don't exactly make sense. Not that they make the best sense in the comics - the Stones are so powerful that they're sort of story-breakers (like, the Mind Gem would allow you to conceivably control the minds of every living being in the universe. That's way overpowered) - but in the movies they've been sort of especially confusing.
And so there's a new featurette as part of the Avengers: Age of Ultron home video release that attempts to lay out the rules and history of the Stones.
I think they did an okay job, especially retconning the Tesseract - actually a version of the Cosmic Cube, which is not an Infinity Gem in the comics - into the Space Stone. Even the Aether makes more sense when looked at through this lens - Malekith was going to replace our reality with his. I'm okay with that! And it becomes clearer when watching this featurette that the Mind Gem created Ultron, not Tony Stark (it took a second viewing of Ultron to really understand that for me - Tony never turns Ultron on, the Mind Stone does).
Where I still draw the line is the Power Stone. It destroys all life, but that has nothing to do with the actual Power Gem's abilities, which give the bearer control over all power and energy in the universe (like I said, these things are so fucking overpowered it isn't funny). Of course the way it's used in Guardians of the Galaxy isn't that much better than how it has been used in the comics - pretty much every dummy who ever got their hands on the device that allows them to control all energy in the universe used it to make themselves really strong.
The featurette draws attention to the fact that the last two Stones are maybe the weirdest and most interesting. The Time Stone is a big one, as it gives the bearer complete control of time and opens the door to lots of time travel shenanigans (like bringing characters back from the dead in Infinity War Part 2). Then there's the Soul Gem, which is really mystical in nature and which, in the comics, is actually sentient and likes to devour souls. If I were a betting man that's the Stone I would expect to show up in Thor: Ragnarok. The Time Stone? I'm not sure. Maybe Infinity War Part 1?
This video does not feature the dopiest thing currently happening in the Marvel Cinematic Universe: the dual Infinity Gauntlets. One popped up in Thor and then we saw another one in Avengers: Age of Ultron, but who cares? The Gauntlet doesn't do anything, it's just a fucking glove on which Thanos mounts the Stones. It has no inherent value or power - it could be the Infinity Tiara or the Infinity Bandoleir or the Infinity Belt or whatever. He could just hold them in his underwear if he wanted. But people keep talking about it like it's a big deal and I'm sort of worried Marvel is going to make it a big deal, which I think would be silly.
Anyway, enough of my nerding. Does this video make the emerging cosmic backstory of the Infinity Stones clearer for you now? Is this stuff going to be too confusing for mass audiences when it comes to Infinity War?
* Christ I hate this word now. It was useful when used in moderation, but it has become such a buzzword when talking about movies that it has lost all meaning.