Abstract Class versus Interface

using System;

namespace AbstractsANDInterfaces
{
	/// <summary>
	/// Summary description for Employee.
	/// </summary>
	
	public abstract class Employee
	{
		//we can have fields and properties in the Abstract class
		protected String id;
		protected String lname;
		protected String fname;

		//properties
		public abstract String ID
		{
			get;
			set;
		}

		public abstract String FirstName
		{
			get;
			set;
		}
		
		public abstract String LastName
		{
			get;
			set;
		}
		//completed methods
		public String Update()
		{
			return "Employee " + id + " " + lname + " " + fname + " updated";
		}
		//completed methods
		public String Add()
		{
			return "Employee " + id + " " + lname + " " + fname + " added";
		}
		//completed methods
		public String Delete()
		{
			return "Employee " + id + " " + lname + " " + fname + " deleted";
		}
		//completed methods
		public String Search()
		{
			return "Employee " + id + " " + lname + " " + fname + " found";
		}

		//abstract method that is different from Fulltime and Contractor
		//therefore i keep it uncompleted and let each implementation 
		//complete it the way they calculate the wage.
		public abstract String CalculateWage();
		
	}
}