Abstract class can contain non abstract method. 
  When we want that all subclass must implement all methods of 
base class ;In such situation use interface.In other hand
when we want only some method of base class in our subclass
then use base class as abstract class because abstract class
can contain non-abstract method.

Interfaces are used when we want classes should follow the 
rules strictly. Rules like naming coventions e.g. if in 
customer class one developer add method AddCustomer other 
developer while implementing Invoice class could write 
method name InvoiceAdd and also could change the Signiture 
rules. for addition of customer we have at client code as 
Cutomer.AddCustomer(A,B,C); and Invoice.InvoiceAdd(A,B)
To maintain uniformality if we declare interface and define 
method Add(A,B) in that interface and implement that 
interface in both the classes then the code will be uniform 
all over the classes where ever we have implemented 
interface and also all concrete classes must contain 
methods which are defined in the interfaces.