- What is an instance?    - Difference between object and instance ?

- You could think of "dog" as a class and your particular dog as an instance of that class.

OR

- A blueprint for a house design is like a class description. All the houses built from that blueprint are objects of that class. A given house is an instance.

if not understanding the above words you can check the below links 

- http://whatis.techtarget.com/definition/instance

- http://stackoverflow.com/questions/3323330/difference-between-object-and-instance