A headless browser is a software that can access webpages without displaying them to the user, allowing it to pipe content to another program. Headless browsers are useful for automation, layout testing, data extraction, and performance testing, as they provide fast feedback and can be used on machines without a GUI. However, they have limitations, such as not being suitable for full UI tests and debugging tools like video validation. A sound testing strategy should include a mix of both headless and real browser testing to ensure coverage and reliability. Headless browsers are particularly useful when performance is critical, as they can provide quicker results without the overhead of starting up a browser GUI.