abstract class contains function prototypes and function
definations also
 but interface ccontains onlu function prototypes which are
used in deriving object declarations

there is not interface and abstrcat keyword in C++.
but in CLR and Java

In relation to C# (not C++):
1. Interfaces are like abstract class where all member 
functions as abstract but cannot have any data members, 
constructors or destructors. Classes using interfaces are 
forced to implement all the methods in the interface.
2. Does not implement any methods. ie only used as a 
template for derived classes to use.
3. All members of interface are public. ie in Interface you 
only have to state long Function(); but in a class you have 
to state public abstract long Function();
4. A class can have multiple inheritence with an interface 
but not a class.
5. Also structures in C# cannot inherit from other classes 
of structures but can implement an interface allowing you 
to implement more features to your class.
By using interfaces you are not making any assumptions 
about the functionality. You want to make sure your classes 
using the interfaces implement the functionality of all the 
methods in your Interface.
With all above in mind you can probably find more uses for 
an interface.
One example: You have many forms of communication like 
FileIO or database or IPC. All these forms of communication 
will need to features like Open and Close either a file or 
a connection Read or write etc. You would then define an 
interface with these features for your concrete classes. 
You can then use the Interface as a base class pointer like 
in C++ to access the derived class's functions.