Unit testing is an essential aspect of software development that ensures individual units of code work as expected by writing test cases to verify their functionality. It's a process that involves separating components of the software, testing them individually, and verifying their performance. Automated unit testing uses tools and scripts to execute tests, providing faster results with less human error compared to manual testing. While manual testing is more flexible and can provide valuable insights, automated testing is ideal for repetitive, time-consuming, or complex tests. The decision between automated and manual testing depends on the requirements and constraints of the testing project. By using the right approach, developers can ensure their software meets high quality standards and catches issues before release.