Class headers are statements at the beginning of each class definition that specify the name of the class, its accessibility (public, private, etc.), whether it is abstract or final, and any interfaces it implements.
Access modifiers determine which parts of a program can access certain variables or methods within a class. Examples include public (accessible to all), private (only accessible within the same class), protected (accessible within subclasses), etc.
Abstract Class: An abstract class cannot be instantiated but serves as a blueprint for other classes to inherit from. It may contain abstract methods (without implementation) as well as regular methods with implementation.
An interface is like a contract that defines a set of methods that implementing classes must provide. It allows for multiple inheritance by enabling a class to implement multiple interfaces.