You form a sphere of fire and autumnal leaves start to form around the fire. The fire can sense if you specifically targeted someone and if you did it will follow them until it hits something or until it's out of your sight at which both yield the result of blowing up. When the fireball ignites it looks as if someone threw a box of flaming leaves at you. 

Each creature in a 20-foot-radius sphere centered on the point of explosion must make a Dexterity saving throw. A target takes 8d8 fire damage on a failed save, or half as much damage on a successful one.

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.