In abstract classes, its upto u to make methods abstract 
but in interfaces, all methods are bydefault abstract.

In abstract class, there can be concrete methods. Whereas it's not the case with interfaces.

Interface is a set of abstract methods,all the methods have 
to be overridden by the class which ever implements the 
interface. 

Abstract class is collection of data and methods,which are 
abstract. In abstact class some methods may contain 
defination,but in interface every method should be abstract
( no definations in the methods)

abstract class                       
1. abstract class have concrete methods ; interface have no 
methods implemented.
2.interface do not come in inheriting chain; abstract class 
come in inheriting chain.
3.in an interface no accessibility modifiers are allowed; 
abstract class may have accessibility modifier

Abstrat Class: It is a combination of Concrete Methods and
Abstract Methods.

Interface: Which class is only having abstract methods is
called interface.

Concrete Methods: Methods having declaration and definition

Abstrat Method: Methods having only declaration not having
any definition. Becuase abstract methods are used to provide
data abstractions(Means providing method definition and
hiding method declaration to provide security to the client).