They are different based mainly on the purity of the silicone they contain.
These two types of solar panels are different based on the purity of the silicon they contain. Monocrystalline cells are purer and thus conduct higher amounts of electricity compared to polycrystalline cells. They are more space-efficient and provide better peak efficiency.
Further, monocrystalline panels are derived from pure drawn crystalline panels and aligned in one direction so that they are extremely efficient when placed under direct sunlight at the correct angle.
On the other hand, polycrystalline cells are comprised of silicone offcuts that are moulded to create a solar cell made of several forms of crystal, which means they are not perfectly aligned. This arrangement results in power loss in the joints of the crystal, translating to lower efficiency, in general. But this misalignment can be an advantage in low light conditions since the cells can absorb light from different angles.