Backbone.js
Backbone.js is a JavaScript Framework that is used to for dividing logic and data from user presentation. Having data intermingled with the DOM is bad practice, Backbone.js aims to make this easier to avoid. Backbone.js provides 5 big things, models, collections, views, routers and events. Backbone.js is best utlilized with data-heavy web applications. Starting development back in 2010, the current version of Backbone is 1.6.0 released in 2024. Backbone needs Underscore.js to work, and jQuery if you need Backbone.View.
Vue.js
Vue.js is a JavaScript framework designed for constructing user interfaces. Vue.js is versatile and flexible and can be applied in many different scenarios. Vue.js is also relatively easy to use and learn, since it's built upon HTML, CSS, and JavaScript. Vue has two core components, declarative rendering and reactivity. Declarative rendering is an addition to normal HTML, allowing for describing HTML output based on JavaScript. Reactivity is a property of Vue, it constantly tracks changes in the JavaScript state and updates the DOM.
Next.js
Next.js is a JavaScript framework that is built upon React, another JavaScript framework. You use Next.js to improve React applications, so it would be unfair to say that Next.js is just React with more features. Next.js has two distinct routers, App Router and Pages Router. App Router is newer and supports newer features of React. Pages Router is older, but still being maintained. To even be able to use Next, you need to understand the React framework first. Next is a powerful tool that greatly expands on React.
Summary of the Documentation
There exists many types of JavaScript Frameworks, each providing their own sets of pros and cons. Depending on the web application you're building, different frameworks exist to help with that. Backbone.js helps with more data-heavy web applications that need the logic and data seperate from user interaction. Vue.js builds complex and good looking user interfaces but could also do many other things. Next.js adds even more features to another framework, React; they co-exist to make complex websites.