Abstract Class versus Interface

using System;

namespace AbstractsANDInterfaces
{
	/// <summary>
	/// Summary description for IEmployee.
	/// </summary>
	public interface IEmployee
	{
		//cannot have fields. uncommenting will raise error!
//		protected String id;
//		protected String lname;
//		protected String fname;

		//just signature of the properties and methods.
		//setting a rule or contract to be followed by implementations.
		String ID
		{
			get;
			set;
		}

		String FirstName
		{
			get;
			set;
		}
		
		String LastName
		{
			get;
			set;
		}
		
		// cannot have implementation
		// cannot have modifiers public etc all are assumed public
		// cannot have virtual

		String Update();

		String Add();

		String Delete();

		String Search();

		String CalculateWage();
	}
}