インターフェースと抽象クラスの違いは何ですか?

interface Drawable {            // a capability
    void draw();
}

abstract class Shape {          // shared identity + state
    protected String color;
    Shape(String color) { this.color = color; }
    abstract double area();     // subclasses must define
    String describe() {         // shared concrete behavior
        return color + " shape, area=" + area();
    }
}

class Circle extends Shape implements Drawable {
    private double r;
    Circle(String c, double r) { super(c); this.r = r; }
    double area() { return Math.PI * r * r; }
    public void draw() { /* render */ }
}