The following needs to be rewritten before being retitled the Definition of this page.
An instance of any thing is one specific Copy of that Type of thing.
When someone says "I modified the Linux kernel to my own tastes" they really mean that they modified an *instance* or *copy* of that software, not that they modified all copies simultaneously.
If someone says "Oh, hey, look, they have the same car as you!" they really mean it is the same type, not that it is the exact same instance.
If you want to modify something, you will usually be modifying that instance.
You can also change the type of that thing to effect *future* copies that are made from that type, but the old type can also be used to make future copies of the old version. This is the essence of forking a design.