What is a memory leak in JavaScript and how can you prevent it?

I'm learning JavaScript performance optimization and often hear about memory leaks causing applications to become slow or crash over time. I understand that JavaScript has automatic garbage collection, so I'm confused about how memory leaks can still occur.

Can someone explain what a memory leak is, the most common causes (such as forgotten timers, event listeners, global variables, and closures), and the best practices to detect and prevent memory leaks? A simple real-world example using browser developer tools would also be very helpful.