Abstract classes define methods and interfaces declare 
methods.
Interfaces declare methods without implementation.
Abstract classes are very much useful when there is a same 
functionality across various classes.
Interfaces are useful for classes which varies in 
functionality and with the same method signature.

A class becomes an abstract class when it has atleast one 
abstract method(a method with no definition).The major 
difference between abstract classes and interfaces is that 
an abstract class can contain both methods with zero 
definition as well as methods with definition.But an 
interface is one in which all the methods are with no 
definition.

Abstract class contains one or more unimplemented methods 
left for future implementation by its derived class.it has 
all property of class it cannot be instantiated.it provides
default behaviour.

Interface is mear specification nothing is implemented its 
just a declaration.in java using interface we can model 
multiple inheritance. it provides design flexibility.

abstraction is used in framework
interface is used in specification

Abstraction is used in framework interface is used in 
specification...

abstraction is used in framework
interface is used in specification