The scripting language for web browsers.
JS History
Before Javascript, the web was static, with pages composed of text and images, styled with CSS.
Javascript was created to make updates to the page without reloading, allowing dynamic UI's and desktop like applications.
Since then, it has evolved into a widely used language on both the front and back end.
ECMAScript is the standard that defines the JavaScript API.
JS Libraries
Libraries are scripts that you use in your own code.
For example, the Swiper library is used for making sliders.
You could build a slider from scratch, but there's a lot to implement, like touch sliding, accessibility, transition animations, etc.
To implement a library read the docs and follow instructions.
Generally, you would add html on the page that follows a specific format, and write some js to target it and load the library.
JS Frameworks
Frameworks are full stack solutions to building applications, using node to run javascript on the backend.
They generally use webworkers in the browser to make dynamic connections to the webserver.