Local and global variables differ primarily in their scope—where in the code they are accessible. A local variable is one that is declared inside a function and can only be used within that function. Once the function has finished executing, the local variable is destroyed and is no longer accessible. Local variables help to prevent naming conflicts and keep different parts of the program independent. For instance, you can use the same variable name in different functions without interference, because each function has its own local scope. Global variables, on the other hand, are declared outside of any function and can be accessed from anywhere in the program, including inside functions. This means that global variables persist throughout the entire program and can be used by multiple functions. While global variables can be convenient for sharing data across different parts of a program, they can also introduce problems if different parts of the code unintentionally modify the global state, leading to bugs that are hard to track down. In JavaScript, if you declare a variable without `var`, `let`, or `const` inside a function, it automatically becomes a global variable, which can lead to unexpected behavior. Managing scope properly is important for writing clean, maintainable code, as it helps to avoid conflicts and ensures that variables are only accessible where they need to be.