There are two types of crystalline solar cells, monocrystalline (single crystal) and polycrystalline.
Monocrystalline cells are made by slicing a single silicon crystal (a boule) into wafers and put into cells. The first silicon solar cells were monocrystalline and over the years, they have become more and more effective at converting sunlight.
These cells are stronger and more efficient than any other types currently on the market, but they are also much more expensive than other types of cells. A major factor in their cost is the fact that silicon boules are circular, so the cells have rounded edges and thus less surface area to absorb sunlight.
Meanwhile, polycrystalline cells are made by melting down purified silicon and pouring it into a square mold. The shape prevents wasted material, meaning finished cells have more surface area dedicated to absorbing sunlight.
Polycrystalline cells are much cheaper than monocrystalline cells, but slightly less efficient because the melted silicon has crystal imperfections within the material. To make up for this, polycrystalline cells tend to be a little bigger than monocrystalline cells.