Theobromine, a constituent of chocolate, is toxic to dogs. The effect on the dog is dependant upon the amount ingested and the body weight of the dog, ie. a small dog will be more affected than a large dog if both eat the same amount.
The reaction will vary between dogs, some only suffering mild diarrhoea whilst it may trigger cardiac irregularity or even epileptic fits or in others. Cardiac arythmia can precipitate a myocardial infarct which can kill the dog. Theobromine also irritates the GI tract and in some dogs can cause internal bleeding which in some cases kills them a day or so later.
Dark cooking chocolate contains the highest amount of theobromine (up to 28mg/g) with the dark continental type chocolate (16mg/g) next, even milk chocolate contains a small amount (2mg/g). Theobromine cannot be easily metabolised and excreted from the dog's system so the effect is cumulative. Theobromine also irritates the GI tract and in some dogs can cause internal bleeding which in some cases kills them a day or so later.
Chocolate Drops made specifically for dogs should, of course, contain little or no theobromine but one has to wonder whether the quality control procedures of all manufacturers of dog chocolates are adequate to guarantee the absence of theobromine and the most prudent policy may well be to avoid giving chocolate of any kind to dogs.
Above all, keep those dark made-for-human chocolates well out of reach. Treat chocolate for what is is to dogs: POISON!!!

FIRST AID: Within two hours of ingestion, try inducing vomiting unless your dog is markedly stimulated, comatose, or has lost the gag reflex. If your dog has eaten a considerable amount of chocolate, or displays any of the above symptoms, take it to the vet without delay.

If the dog has not already vomited, induce vomiting by giving 1 tablespoon of 3% hydrogen peroxide per 20 pounds every 10 minutes until vomiting starts. (No more than three times.) If no vomiting occurs within 30 minutes, transport the dog immediately to the veterinarian. 
Now there might be a chance that you don't have hydrogen peroxide in the house. In that case you can use salt water. (I always keep a jar of sauerkraut in the fridge - that usually will make a dog vomit as well.) 
In the absence of major symptoms, administer activated charcoal. The unabsorbed theobromine will chemically bond to this and be eliminated in the feces. In pinch, burnt (as in thoroughly burnt, crumbling in hand) toast will do.