What is the difference between an interface and abstract class?

Hi..

here is the link to give brief difference
http://www.dotnetbull.com/2011/11/difference-between-abstract-class-and.html[^]

Abstract class:

a) An abstract class is a special kind of class that cannot be instantiated.
b) It allows the other classes to inherit from it but cannot be instantiated.
c) It is used to carry the same hierarchy or standards for all the subclasses.
d) A class can inherit an abstract class without implementing all its abstract method.
However only a class that has all its method implemented can be instantiated to an object.
IS-A relationship.
e.g. Student IS A Person, Employee IS A Person.

example

// 'framework library' for a person
// a person can enrol and submit
// however, the class that consume this framework library
// need to provide 'where' the paperwork need to be sent
public abstract Person
{
    public abstract SendPaperWork(string paperwork)

    public void Enrol()
    {
        SendPaperWork("enrolment");
    }

    public void Submit()
    {
        SendPaperWork("report");
    }
}

// by inheriting Person abstract class
// we are enabling student to enrol and submit
// however, SendPaperWork need to be implemented
// because we need to tell it explicitly 'where' 
// to send the enrolment/ submission
public class Student : Person
{
    public override SendPaperWork(string paperwork)
    {
        School.Send(paperwork);
    }
}

// an employee send the paperwork to a different 'place' than student
public class Employee : Person
{
    public override SendPaperWork(string paperwork)
    {
        Company.Send(paperwork);
    }
}

Interface:

a) An interface is not a class,but it is an entity and has no implementation.
b) It has only the definition of the methods without the body
c) A class that implements an Interface need to contain all the implementation, otherwise the compiler will throw an error.
CAN-DO relationship.
e.g. Student CAN enrol, Student CAN submit assignment.

example

public interface ICanEnrol
{
    void Enrol();
}

public interface ICanSubmit
{
    void Submit();
}

public class Student : ICanEnrol, ICanSubmit
{
    public void Enrol()
    {
        School.Send("enrolment");
    }

    public void Submit()
    {
        School.Send("report");
    }
}

public class Employee : ICanEnrol, ICanSubmit
{
    public void Enrol()
    {
        Company.Send("enrolment");
    }

    public void Submit()
    {
        Company.Send("report");
    }
}

public class MailServer
{
    public void SendAllSubmissions()
    {
        // AllSubmitters is a collection of students and employees
        foreach (ICanSubmit submitter in AllSubmitters)
        {
            // The MailServer does not care if 
            // the submitter is a student
            // or an employee, as long as it can submit
            submitter.Submit()
        }
    }
}

Thank You
Happy Coding