What is the difference between pass by value and pass by reference?

Pass by value creates a copy of the argument; changes inside the function don't affect the original. Pass by reference passes the memory address; changes inside the function modify the original. Java is always pass by value, but for objects, the reference is passed by value (can modify object's state but not reassign reference). C++ supports both explicitly with & for reference. Python passes object references by value (mutable objects can be modified, immutable cannot). Understanding this prevents bugs with unintended modifications.