Internal Energy (U): This term refers to the total kinetic and potential energy contained within a system - think about this as your cash on hand.
Pressure-Volume Work (PV work): This term describes work done by or against pressure forces during volume changes - akin to investing money where there are risks but also potential gains.
Thermodynamic System: A thermodynamic system includes everything within defined boundaries where we study energy transformations - just like considering all your financial assets together forms your financial system.