Mono crystalline silicon panels are made of cells that have been grown from a single crystal and poly-crystalline is more of a cast process. Technically the mono-Si cells are more efficient since the crystal structure is continuous but new technology has narrowed the gap considerably. Mono-Si may still command a bit of a premium but there is not much difference today. Mono-Si panels will appear black and poly-Si will appear dark blue. Sometimes mono-Si panels with black frames and black back sheet are specified because they just look better.