a one piece wheel is made from a single piece of metal through either forging or casting.
a two piece wheel is a wheel face bolted to a rim with hardware. The same face can be used with different width barrels to change the wheel fitment and offset with only minor changes. technically you could just replace the wheel barrel if you damage it by curbing or bending, although in some cases it's almost as costly as replacing the entire wheel. Most people running 2 or 3 piece wheels (3 piece wheels consist of the wheel face and an inner and outer barrel piece sandwiched together) purchase them for aesthetic reasons.
2/3 piece wheels aren't necessarily better. it's entirely dependent on the application. And in many cases a one piece is preferred for weight savings.