Override & new keyword

Question

I wish to know different between using the new and the override keyword in this program. Because both are giving same results in following program. Even though it is said that Tostring() overrides the Object class version. You can notice in this text there is an override word but in the example program new is used.

Also when return keyword is used in the method header, indicates the type of return. But in this program in the method header type is string, return is combination of string and integer. Please explain the reason.

using System;
class DemoStudents4
{
public static void Main()
{
Student payingStudent = new Student();
ScholarshipStudent freeStudent = new ScholarshipStudent();

payingStudent.SetName("Megan");
payingStudent.SetCredits(15);

freeStudent.SetName("Luke");
freeStudent.SetCredits(15);

Console.WriteLine(payingStudent.ToString());
Console.WriteLine(freeStudent.ToString());

Console.ReadKey();
}
}

class Student
{
private string name;
protected int credits;

public void SetName(string name)
{
this.name = name;
}
public void SetCredits(int creditHours)//SetCredits in the child class as well
{
credits = creditHours;
}
public new string ToString() //Tostring() overrides the Object class version.
{
string stuString = "Student " + name + " has " + credits + " credits";
return stuString;
//OR
//return String.Format("Student {0} has {1} credits", name, credits);
}
}

class ScholarshipStudent : Student
{
new public void SetCredits(int creditHours)
{
credits = creditHours;
}
}
/*
Student Megan has 15 credits
Student Luke has 15 credits
*/