Heh...it's a nice post and an honest question, but...is this really your first encounter? It's almost as if you've been living under a rock for almost four or five years.
No, this is certainly NOT comparable. While both are meant to get the word out for the game, they do so in very different styles. In fact, the "getting the word out" is perhaps the only similarity.
Demo's are when parts of a game that is "done" (or 'gone gold', to use the proper term) are publicly released for the general audience for free. This gives them an idea of what the game looks like, how it plays and whether or not it plays smoothly.This might be only a few levels, a few characters of the total roster, or even have some important parts (weapons) missing.
Early access games aren't done yet. You pay either full price or a part of it(1), but in a state that might not represent the game how it ends up being. The idea is that developers listen to suggestions and crowdsource bug-finding, whereas eager gamers can get their hands on games before the "main audience" gets it.
The definition of "a state that might not represent the game" is a bit of a dodgy one, though. Some developers release an open access the moment they've got a working beta version, others when they've squished all bugs they've found and want to test the audience's waters early.
A few examples from my own experience:
-factorio has been in early access for nearly ten years. The devs have always added and tweaked their game, so by the time I got in (only a few months before actual release) I couldn't get it to crash no matter what I tried. The fact that people have put many tens of thousands of gaming hours into it also meant the thing was as stable as it could be
-parkitect was a fun one to watch develop (it was a rollercoaster tycoon kind of game). Nearly every week, there were blog posts of what the developers were doing, what was added, tweaked or fixed. I (finally) got the game well after a year of such weekly updates, but the game was still quite barren. That is to say: it had just a sandbox mode where you could plop down rides and 'somewhat' monitor your peeps. The whole scenario and premade parks were added much later. Rides and stalls were also only gradually added, though it had quite a decent selection when I got around to buying it.
-cook, serve, delicious 2 was...a different kind of early access. I can't really remember how I got in touch with the (sole) developer, but we mailed each other even before he came around to publicly announcing the sequel to CSD 1 (which I absolutely adore). However, it quickly became clear that when he had his group of testers, he had done most of the ground work. This isn't to say we weren't influential, but it meant actively searching for bugs and QoL improvements that might or might not make the cut.
In any case: I would look at early access games individually. I've read 'reviews' on quite a few games that were abandoned before they ever really got to a state of being ready. You also pay full price for a game of which you know will be of lesser quality than upon release. So I'd only buy it if A) you are certain that the game is of decent stable quality (factorio is the de facto example), B) you're a big fan of what the developers want to achieve (CSD 2, in my case), and/or C) you've got enough trust that they keep working at it until release (the weekly updates of parkitect drew me over the line).
Ideally, I'd go only for it if at least two of those match,but ey...it's your money.
Nope. As outlined earlier: you pay for the full game. If you want, you can even buy the game and then leave it in your backlog until it "properly" releases. In a way, you lend the developer the cost to create the game. He just repays with the game, that's all.
(1): I've seen games that were in development for years and gradually became more expensive. Or from the other perspective: the earlier you buy the game, the cheaper you'll get it.