Unit testing in Node.js is a crucial practice that helps developers identify bugs, streamline debugging, and facilitate code refactoring by testing isolated units of code. The guide emphasizes the importance of selecting the right testing framework based on various criteria such as ease of setup, support, feature sets, speed, reporting, and integration capabilities. It highlights popular Node.js testing frameworks, including Mocha, Jest, Jasmine, and AVA, each with unique strengths and weaknesses. Mocha is praised for its extensibility and plugin support, while Jest offers comprehensive documentation and optimal performance with parallel test running. Jasmine is recognized for its simplicity and long-standing community support, while AVA is noted for its minimalism and speed with parallel tests. Additionally, the guide covers other libraries like Chai for assertions, Sinon for creating stubs and mocks, Supertest for HTTP testing, and Webdriver for browser automation. Ultimately, the choice of framework should align with the specific needs and future requirements of the project.