Access modifiers control visibility: private (class only), package-private/default (package only), protected (package + subclasses), public (everywhere). Packages organize related classes and prevent naming collisions. Understanding access control is essential for writing maintainable APIs.
private: Your personal diary. package-private: Files in your home office. protected: Your family car (family + guests). public: Your published book.
What is 'package-private' (default) access and when would you use it?