Testem is a test runner that makes JavaScript unit testing easy. It supports various testing frameworks like Mocha, Jasmine, and QUnit. It provides features like automatic browser launching, test reloading on file changes, and a customizable reporter. Testem focuses on fast feedback and supports continuous integration environments, making it a versatile tool for any JavaScript project. Easy to configure and highly adaptable, Testem provides a solid foundation for client-side JavaScript test automation.