The 3 main modern competitive cell types are Monocrystalline, Polycrystalline and Amorphus. They all have their advantages and disadvantages. Listed below are the basics to help you choose.
Monocrystalline (single flat dark blue colour) has a typical efficency rating of about 15-17%. Costs are usually slghtly higher than other types due to complicated manufacturing techniques.
Polycrystalline (lighter blue broken glass appearence) is similar to monocrystalline except the slices are made from multiple crystalls grown together. Costs are slightly lower and efficiency is around 12%. These panels suffer from efficency loss at high temperature.
Amorphous (brown gloss as seen on calculators) operates at around 6% but has the advantage of being flexible. Larger panels tend to be more expensive than crystalline versions as they are less popluar.