A runtime error occurs when a program is running, causing it to stop unexpectedly. These errors can happen for various reasons, such as trying to divide by zero, accessing a file that doesn't exist, or using a variable that hasn't been initialized. Unlike syntax errors, which are detected before the program runs, runtime errors only appear during execution. Developers often use debugging tools to identify and fix runtime errors. By analyzing error messages and the program's behavior, they can locate the source of the problem. Proper error handling techniques can also help manage these issues, ensuring that the program can recover gracefully instead of crashing.