Mocha and QUnit are both popular JavaScript testing frameworks used to perform unit testing in web applications. While they serve the same purpose of automating the testing process, there are several key differences between Mocha and QUnit.

1. Structure and syntax: Mocha provides a more flexible and expressive syntax for writing test cases and supports various styles such as BDD (Behavior-Driven Development) and TDD (Test-Driven Development). On the other hand, QUnit follows a more traditional and simplistic approach with a structured syntax.

2. Asynchronous testing: Mocha is known for its excellent support for asynchronous testing. It provides built-in mechanisms like promises and callbacks to handle async operations effectively. QUnit, on the other hand, requires explicit handling of asynchronous operations using its built-in special methods like assert.async().

3. Test runner: Mocha provides a feature-rich test runner that runs tests in the browser, Node.js, or even on the command line. It allows you to specify the desired environment and provides a wide range of reporting options. QUnit, however, primarily focuses on browser-based testing and doesn't offer as many options for running tests in different environments.

4. Plugin and integration ecosystem: Mocha has a vast ecosystem of plugins and integrations with various libraries and tools, making it highly customizable and extensible. Conversely, QUnit has a smaller plugin ecosystem and is generally integrated with other frameworks like jQuery.

5. Assertions: Mocha relies on external assertion libraries like Chai for performing assertions in tests. This allows developers to choose from a wide range of assertion styles and libraries. QUnit, on the other hand, has a built-in assertion API that covers most use cases, making it easier to get started without any additional dependencies.

6. Community and adoption: Mocha has a large and active community with widespread adoption in both open-source projects and commercial applications. It is often the preferred choice for developers due to its flexibility and extensive features. QUnit, while still popular, has a relatively smaller community and is commonly used in projects that heavily utilize or are built on the jQuery library.

In summary, Mocha provides a more flexible syntax, extensive asynchronous testing support, and a rich plugin ecosystem, while QUnit offers a simpler syntax, built-in assertion API, and close integration with jQuery. The choice between Mocha and QUnit largely depends on the specific requirements and preferences of the project and development team.