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. Common programming languages like Python, Java, and C++ provide error messages that help pinpoint the issue. Addressing these errors is crucial for ensuring that software runs smoothly and reliably.