An interface is purely contractual in that it only defines
method headers but no implementation. Classes that implement
the interface must provide implementation to all method
defined by the interface.

An abstract class contains partial implementation, i.e. it
provides implementation for some methods where as just
defines abstract method headers for other. Abstract classes
cannot be instantiated directly and have to be extended by
subclasses who implement the abstract methods defined in the
abstract superclass. Unlike an interface, subclasses of the
abstract class only need to implement only those methods
that are marked abstract.

A programer uses an abstract class when there are some 
common features of the program shared by all the objects.
A programer uses an interface when all the features of the 
program are implements differently for different objects.