Interface

Simple to understand  Object is belongs to which category.Such as person belong person class, car belong car class.An abstract class is virtual, objects is specific.
In general, if use the abstract modifier class indicates that it will serve as a base class.

using System;
namespace ConsoleApplication1
{
    abstract class Person  // Person is an abstract class.it as base class
    {
        public abstract void Name(); // In a derived class must implement it
        public abstract void Work();
    }
    class Ken : Person
    {
        public override void Name() // Use the "override" keyword implement abstract method in the base class
        {
            Console.WriteLine("My name is Ken.\n");
        }
        public override void Work()
        {
            Console.WriteLine("My work is Student.\n");
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Ken k = new Ken();
            k.Name();
            k.Work();
        }
    }
}

An abstract class can also implement polymorphism:
using System;
namespace ConsoleApplication1
{
    abstract class Employee
    {
        protected string _name;
        protected Employee() { }
        protected Employee(string name)
        {
            this._name = name;
        }
        public abstract void Work();
    }
    class Manager : Employee
    {
        public Manager(string name):base(name){}
        public override void Work()
        {
            Console.WriteLine(_name +" in analyzing the stock market...\n");
        }
    }
    class Engineer : Employee
    {
        public Engineer(string name) : base(name) { }
        public override void Work()
        {
            Console.WriteLine(_name+" Writing code...\n");
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Employee[] emp = new Employee[2];
            emp[0] = new Manager("James");
            emp[1] = new Engineer("Ken");
            Console.WriteLine("Start working");
            foreach (Employee t in emp)
            {
                t.Work();
            }
        }
    }
}

Hi Srikant,

Refer to this post and read it carefully, this answers your question:

http://www.headspring.com/two-reasons-to-use-abstract-classes-in-c/

Also, you may want to Google it.

"Basically, Abstract classes help you improve your code not only through what they can do, but also through what they can't. They can hold common features for many classes to inherit, without accidentally becoming objects themselves. They can also force you to write unique implementations of a common method, preventing human errors of forgetfulness. When you set up a class hierarchy, seriously consider whether you need a normal class to inherit from, or whether an abstract class will do. It may end up making the difference between a solid or buggy program." 


Happy Learning :)
Cheers!