Ember.js is a JavaScript framework for building web applications. It is based on the Model-View-Controller (MVC) pattern and is designed to be used with the Ember CLI, a command-line interface for building Ember applications.

One of the key features of Ember is its focus on conventions and best practices. This makes it easy for developers to quickly build complex web applications, as the framework provides a clear structure and set of guidelines for building and maintaining your code.

Ember also has a strong emphasis on performance, with features such as lazy loading and a powerful router that allows for the efficient rendering of complex pages. This makes it well-suited for building large-scale applications with many routes and components.

In terms of comparison with other JavaScript frameworks, Ember can be seen as a more opinionated and convention-driven framework compared to something like Angular or React. This means that while it may have a steeper learning curve, it also provides a more structured and predictable development experience. Additionally, Ember has a strong focus on performance, which sets it apart from some other frameworks that may prioritize ease of use or flexibility.

Another key difference between Ember and other JavaScript frameworks is that Ember is a full-stack framework, meaning it provides everything needed to build a complete web application, including a built-in data layer and templating system. This can be a benefit for developers looking to build complex, data-driven applications, as it eliminates the need to integrate with additional libraries or frameworks.

Overall, Ember.js is a great choice for building large-scale, complex web applications. Its focus on conventions and performance, along with its full-stack capabilities, make it a powerful tool for building high-quality, maintainable code. While it may have a steeper learning curve compared to other JavaScript frameworks, the structured and predictable development experience it provides can be well worth the investment for experienced developers.