Ember.js is a JavaScript framework for building single-page web applications using MVC architecture. It allows developers to build applications with less code through features like handlebars templates. Ember.js makes building JavaScript apps simple by handling tasks like routing and simplifying common patterns. Developers can learn to use Ember.js through their comprehensive guides on the website, which cover everything from basics to advanced concepts. The framework is open source, free to use, and can be downloaded from their website.