Basically there is no implementations in an Interface.While we using an interface , you should implement the methods/properties inside your class.i.e;an interface method is doing nothing until you implemented in your class.The implementation class is responsible for the total implementation in case of an interface.
An absract class is a base class that behaves like a normal class except these have no independent existance, i.e;it is not possible to create an instance of an abstract class directly.The instance can be created using the derived class only.The methods/properties may/mayn't be implemented in the derived class except for abstract members.The abstract members should be implemented in the derived class.
Thus an interface looks similar to an abstract class with only abstract members.
public abstract class SampleAbstract
public abstract void Show();
public abstract void Calculate();
is almost similar to
public interface SampleInterface