Both forged and cast wheels start as aluminium, but they do not behave the same way once they are on the car.
How they are made
A cast wheel is poured into a reusable steel mould. A forged wheel is pressed from a billet of solid aluminium under several thousand tonnes of pressure, then machined. Forging aligns the metal grain into the shape of the wheel, so load travels through the part along the direction the metal was worked.
Weight
An equivalent forged wheel typically comes in around 25 percent lighter than a cast wheel of the same size and design. That is unsprung mass removed from each corner, which is where a lot of the real-world benefit comes from.
Strength
The refined grain structure means a forged wheel absorbs an impact better and returns to shape rather than cracking. This matters for kerbing, potholes and track use.
Finish
Machining a face allows far more precise finishing than a painted cast surface. Two-piece and three-piece forged builds also let each section carry a different finish, which is where most of the visual complexity in the aftermarket comes from.