What is the difference between a square shoulder guitar and round guitar?

There are no guitars with round bodies. This is most likely not what you are asking about. Perhaps you are referring to a classical shoulder and square shoulder dreadnought guitar.

Round shoulder guitars are typically constructed with a round soundhole and a body that is gently curved on the back and sides. Typically the lower bout is 15 1/2" and the upper bout is 11". The design emphasizes overtones and is commonly associated with the dreadnought body.

Square Shoulder Guitars are typically constructed with a square soundhole and a body that is flat on the back and sides. They typically have a wider string spacing at the saddle than round shoulder guitars, allowing the strings to vibrate more freely and produce a louder, more resonant sound. The design emphasizes the fundamentals and is commonly associated with the Martin dreadnought. The lower bout is 16" and the upper bout is 12".