Mainly there are four concepts for OOAD.
1.Abstraction -grouping of data and behaviour(operations) 
into a conceptual object.
2.Encapsulation -Its the implementation mode of data 
abstraction.With encapsulation, we can accomplish data 
hiding
3.Polymorphism -Polymorphism refers to the process whereby 
an object invokes a method of another object in a common 
manner (with the same name) without understanding or caring 
how it is accomplished. 

4.Inheritance-When an object is derived from another 
object. It inherits all the properties etc

Abstraction - Interfaces are the best examples of 
abstraction .
Encapsulation - Classes are best examples of encapsulation.

Generally we encapsulate what varies & 
we abstract commonalities...

One more thing, You should always subclass for behabiours..
you should never subclass for properties / attributes. If 
only properties vary, then you have got to encapsulate them 
in somewhere else e.g. in a class or in some collection 
object like - Properties in Java / Map 

Hope this helps..