(I am not affiliated with any of these technologies, but am a Next/Vercel customer. I am also not super familiar with anything except Next, but this is my attempt at an explanation.)

I think they all try to solve the same problem: how to get a modern interactive app to run on (and be performant) what is essentially a hacked-together ecosystem, HTML + Javascript, with decades of backward compatibility baggage. The essential problem is that browsers work on the ancient and really poorly designed DOM, but developing against the raw DOM sucks. It's fine if you have a simple webpage with headers and some text, but once you get into stateful UIs, it gets hard to maintain pretty quickly. So there's a mismatch between user experience (in HTML) and developer experience (terrible in HTML, better in other frameworks). So developers of complex apps end up abstracting it away with something like a JAMstack.

So you have things like React, which is essentially a UI library (vs a more fully-featured framework like Angular or even the older Rails stuff, or something like Laravel/Symfony for PHP or whatever the .NET equivalent is). React lets you compose apps not out of DOM primitives but components you define yourself, which in turn are reusable and composable.

But there's a lot of things that React don't handle out of the box: page routing, state persistence, static builds, image optimization, hot reloads, CDN caching and invalidation, etc. A lot of teams end up reinventing all those wheels, or else clobbering together 80 different open-source solutions and 10 vendors. It gets hard to maintain very quickly.

Enter Next.js, one of the earlier successful React-based frameworks. It turns a React app from a quirky UI library into something almost beautiful, because you can now make an entire app, not just a UI, using React and some easy to learn JS config objects.

For example, to make a blog with React, you'd first need a CMS (let's assume you have that part figured out) and an API (also figured out). You can write it as a single-page app, using fetch() or whatever to query the API every time. But then the client has to download that and then render the page. If the CMS is on a different host than your webpages are, it can take quite a while. That whole time your user is waiting, seeing a blank page. And if your CMS goes down, your website goes down, even if the content's been the same for days.

Anyway, you could try to statically bake all that into HTML, but then every time you add a new blog entry or update an existing one, you have to rebuild your project. And then if you want it to be fast, you have to invalidate all your CDN caches.

Next.js essentially takes care of all of that for you, in one easy to use and well documented package. Combined with Vercel (the company behind Next.js, who provides hosting) it also abstracts away all the complexities of the buildchain, CDNs, invalidations, etc.

As a duo, their most powerful feature is rehydration. You can code your app as though it were a single-page app, using React to compose components and pages, combined with file-system routing, to create a whole site. But then you push your changes and that's where the magic starts: Your Next.js server (like Vercel) picks it up, builds it with data fetched server-to-server from the CMS, bakes everything into flat HTML + CSS, and invalidates it across the CDN within seconds. At this point, any user who visits your site will be able to download the HTML + CSS, even with Javascript disabled -- the client does not ever speak to your backend directly. To them, your page is just a static HTML page, served straight from the CDN edge. This means the client doesn't need to load React to see your page. They can have JS disabled and it still shows up normally, it just won't be interactive.

Seconds later after the HTML has loaded, some "bootloader" JS then downloads all the other JS that enables interactivity and dynamic data fetches (comments, etc.)... all invisibly to the user. That is the "rehydration", taking a React app that you wrote and the server buildchain "dehydrated" (baked into HTML + CSS), but then rehydrating it to add interactivity back. Yes, you could do all that manually, but Next.js makes it magically trivial... you never have to think about it, it just works. And it's lightning fast.

So take that rehydration stuff and add on a bunch of other quality-of-life developer experience improvements. For example, images are traditionally another headache, needing something like Imgix or Cloudinary to be able to dynamically resize them on the server (so the el cheapo 320p Android doesn't get served the same 4k retina image). Same with script updates... if you change your SPA, you have to figure out how to invalidate current caches, how to sure visitors who've cached the old version still works with your backend API, etc. Or hot refreshes, or page-by-page invalidations, or whatever. It does all of this in one framework, so you can get rid of ReactRouter, Redux (useContext can handle many uses cases), image processors, Preact, Express, etc. Really the only thing you need to provide is a CMS of some sort, typically a headless one.

That's Next.js (the open-source framework). Behind them is Vercel (the hosting/PaaS company which maintains Next.js). Next is the one I'm most familiar with, but I believe the others are similar (but someone correct me if I'm wrong):

Nuxt.js is to Vue what Next is to React. I believe it's a little less featureful than Next, and it's maintained by a different company.

Remix takes some of the Next.js principles but implements them differently; instead of having a server rebuild and bake your project at push, you can do a similar thing on edge compute/serverless (like Cloudflare Workers directly). It has a really neat feature: nested routes (really more like UI layouts), which are composable UI units that are hydrated serverside, similar to Next.js, and then sent to the client as HTML wholesale. So like your <Dashboard> can include <Widget> and <Chart> and <Comments>, but each one can be individually hydrated, composed, and reused -- all invisibly to the client. My understanding (again, not familiar with Remix) is that this was such an amazing feature that Next.js straight up copied it last month with their Layouts RFC (https://nextjs.org/blog/layouts-rfc?utm_source=next-site&utm...).

Fresh looks like Deno's attempt to produce something similar, but it's still early and not quite as powerful.

If you're a web dev and you've never tried this stuff, I strongly recommend taking a look. I've been coding webpages since Netscape, before CSS was invented. Next.js was the single biggest improvement to my professional life in decades, especially coming from the hell that was Drupal + jQuery. I coupled Next/Vercel with a vendor-supported headless CMS, and the overall developer experience made me fall in love with my job for the first time ever. So much so, I actually switched careers from being full-stack to solely frontend/React focused, because Next.js just made it so enjoyable. No more infra and DevOps hell, it all just works, it's all in JS/Typescript, and I can just focus on coding for UX. What used to take weeks to do in the old PHP + jQuery framework would only take minutes to prototype, a day or two to finish in Next. I hope the other frameworks bring you as much joy!