The difference between is and as operator is that:

An object can be cast to a certain type then is returns true otherwise false, but talking about as operator, it is used to cast an object to a specific type but if in any case it is unable to type cast that object, it returns Null.

Ex.

    class Student
    {
        public int RollNumber { get; set; }
        public string Name { get; set; }
    }

    class RegularStudent : Student
    {
        public int MonthlyAttendance { get; set; }
    }

    class CorrespondenceStudent : Student
    {
        public int AnualAttendance { get; set; }
    }



    protected void checkIs()
    {
        Student student = new Student
        {
            RollNumber = 123,
            Name = "Gaurav"
        };

        // this will return true
        if (student is Student)
        {
            Response.Write(student.Name + " is a student");
        }
        else
        {
            Response.Write(student.Name + " is not a student");
        }

        //  this will return false
        if (student is RegularStudent)
        {
            Response.Write(student.Name + " is a regular student");
        }
        else
        {
            Response.Write(student.Name + " is not a regular student");
        }
    }

    protected void checkAs()
    {
        Student student = new Student
        {
            RollNumber = 123,
            Name = "Gaurav"
        };

        RegularStudent regularStudent = student as RegularStudent;

        // this will return true
        if (regularStudent == null)
        {
            Response.Write("Student is not a RegularStudent");
        }
        else
        {
            Response.Write("Student is a RegularStudent");
        }

    }

Note : RegularStudent class is a derived type and the Base types to Derived type casting is not allowed but vise versa is possible.