The main difference between mono-crystalline and poly-crystalline panels is the purity of the silicon used.
To make solar cells for monocrystalline solar panels, silicon is formed into bars and cut into wafers. These types of panels are called mono-crystalline to indicate that the silicon used is single-crystal silicon. Because the cell is composed of a single crystal, the electrons that generate a flow of electricity have more room to move. As a result, monocrystalline panels have greater efficiencies than their poly-crystalline counterparts and generally thought of as a premium solar product.
Poly-crystalline solar panels generally have lower efficiencies than mono-crystalline panels, but their advantage is a lower price point. Poly-crystalline panels are also made from silicon, however instead of using a single crystal of silicon, manufacturers melt many fragments of silicon together to form the wafers for the panel. These panels are also referred to as multi-crystalline because there are many crystals in each cell, meaning there is less freedom for the electrons to move.
Aesthetically mono-crystalline panels are favoured by customers for their black appearance opposed to the poly-crystalline panels, which are bluer in colour due to light reflection.