The article explains that chocolate contains theobromine, an alkaloid that dogs can't metabolize quickly. If a dog ingests a large amount of chocolate, the theobromine can build up and reach a toxic level of concentration, leading to cardiac arrest. Different types of chocolate are more harmful than others, and the effects will vary, depending upon the size and weight of the dog. However, it is potentially lethal.
If you know your dog has ingested chocolate (signs of chocolate poisoning include vomiting, diarrhea, excessive urination, a quick heart rate, and rapid breathing) take him to a vet immediately.