What exactly is a state variable in thermodynamics? I have heard that temperature is a state varibale because we are not interested in how this state is achieved but how is the work done by a system not a state variable? also, the book I'm having says that "the difference between 2 quantities which are now state variabales is a state variable itself." what exactly does it mean? The book only has one paragraph dedicated to this. And I'm confused now.