Chocolate contains methylxanthines which consists of caffeine and theobromine. It is exactly this component to whom dogs are very sensitive to. The amount of methylxanthine in chocolate it varies, because of so many different types of chocolates out there.
Darker chocolates are more pure and dangerous for a dog to consume than a milk chocolate can ever be. With humans it is different, due to our ability to break down and excrete theobromine much more efficiently than a dog ever can.
If your dog happens to ingest chocolate call immediately the clinic for professional assistance. If it is treated before the symptoms of the poisoning show it is much more easy to recover from it. In any case if your dog is showing any of the symptoms above you need to seek help urgently. The symptoms might be vomiting, high body temperature, diarrhea, muscle rigidity, low blood pressure, increased reflex response, rapid breathing, increased heart rate and seizures.