In Java, a superclass is a class that is inherited by another class, which is called a subclass. The subclass inherits all the members, including fields, methods, and nested classes, from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

The Object class is the root of the hierarchy of classes in Java, and every class is implicitly a subclass of Object. If a class does not explicitly inherit from another class, it inherits from Object. The Object class defines and implements behavior common to all classes, including the ones that you write.

The keyword "super" is used to access superclass members inside a subclass. The members of a superclass are constructors, attributes, and methods. The super keyword can be used to call a superclass constructor, access a superclass method or variable, or invoke a superclass constructor with arguments.

Inheritance is a useful feature in Java because it allows coders to reduce the amount of code they have to repeat by using existing code to create new classes. It is useful for code reusability, as it allows attributes and methods of an existing class to be reused when creating a new class.