In Java, an abstract class is a class that cannot be instantiated, but can be used as a base for other classes. Abstract classes are designed to be extended and serve as a blueprint for other classes to implement the functionality of the abstract class.

An abstract class can contain both abstract and non-abstract methods, but any method declared as abstract does not have a body, only a method signature. This means that the subclass that extends the abstract class must implement the abstract methods. Non-abstract methods, on the other hand, can be used as is or overridden by the subclass.

Another important feature of abstract classes is that they can have instance variables and constructors. However, since abstract classes cannot be instantiated, constructors are typically only used to initialize the instance variables of the class.

To create an abstract class in Java, the keyword "abstract" is added to the class definition. Here is an example: 

public abstract class Animal {
    public abstract void makeSound();
    public void eat() {
        System.out.println("Animal is eating");
    }
}

In this example, the Animal class is an abstract class with an abstract method makeSound() and a non-abstract method eat(). Any class that extends Animal must implement the makeSound() method, but can use the eat() method as is or override it.

Overall, abstract classes are useful for creating hierarchies of classes where some of the methods and properties are shared across multiple subclasses.