A solar panel is made using solar cells. Each cell is about 6 inches, 125mm, square. The cells are made using one of two different materials, monocrystalline or polycrystalline.
Polycrystalline is slightly cheaper but is less efficient so a solar panel made with monocrystalline cells will generate more energy than one made using polycrystalline cells, but will be more expensive.
Monocrystalline cells are also darker and so best used when you want a black back sheet for the best looking solar panels.