NET - What are User-defined types in .NET? How to create user-defined in C#.NET?

Public class Student
{
       int age;
       string name;
       public Student(int _age, string _name)
       {
              age=_age;
              name=_name;
       }
       public int Age
       {
              get{return age;}
              set{age=value;}
       }
       public String Name
       {
              get{return name;}
              set{name=value;}
       }
}