There is a difference in the manufacturing technique, and as a result, monocrystalline cells are more efficient. The monocrystalline cells can produce more power in a smaller space. Also, monocrystalline panels have a darker blue color, almost appearing black, and are usually higher in price due to the increased efficiency.
Polycrystalline panels, on the other hand, are less efficient and require more space. However, the difference is size and efficiency do not impact power. A 100 Watt monocrystalline panel and a 100 Watt polycrystalline panel will both output 100 Watts, neglecting power loss in the system, but a monocrystalline panel will output 100 watts with a smaller surface area.