inctance and object of a class.

There's no difference between an object and an instance of a class - they're just different names for the same thing.

However, in the case of structs, instance is the correct term. As structs are value types, it's not really correct to talk about an object of a struct type.

http://en.wikipedia.org/wiki/Instance_(computer_science)

http://en.wikipedia.org/wiki/Object_(computer_science)