Difference Between this and this() in Java

import java.io.*;

public class Student {

    private String name;
    private int age;

    // Note that in the Constructor below "this keyword" is
    // used to differentiate between the local variable and
    // the instance variable.

    public Student(String name, int age)
    {
        // Assigns the value of local name variable
        // to the name(instance variable).
        this.name = name;
        // Assigns the value of local Age variable
        // to the Age(instance variable).
        this.age = age;
    }

    public void show()
    {
        System.out.println("Name = " + this.name);
        System.out.println("Age = " + this.age);
    }

    public static void main(String[] args)
    {
        // Creating new instance of Student Class
        Student student = new Student("Geeks", 20);
        student.show();
    }
}

import java.io.*;

public class Student {

    private String name;
    private int age;

    // Constructor 1 with String as parameter.
    public Student(String name)
    {
        // This line of code calls the second constructor.
        this(20);
        System.out.println("Name of Student : " + name);
    }

    // Constructor 2 with int in parameter.
    public Student(int age)
    {
        System.out.println("Age of student = " + age);
    }

    // Constructor 3 with no parameters.
    public Student()
    {
        // This line calls the first constructor.
        this("Geeks");
    }

    public static void main(String[] args)
    {
        // This calls the third constructor.
        Student student = new Student();
    }
}