Yes, chocolate is toxic to dogs.
The higher the cocoa content the more toxic it is. Therefore, less amount of 80% dark chocolate needs to be consumed compared to white chocolate before toxic signs can be appreciated.
Also, the weight of the dog is important in terms of the amounts needed to be ingested before toxic symptoms appear. Less than 1oz of milk chocolate per 1lb of body weight needs to be eaten to be potentially lethal and less than 0.1oz per 1lb of baking chocolate is needed to be potentially fatal. If you are unsure, you should always contact your vet or emergency vet straight after ingestion.
If your dog has eaten more than the toxic amount, vomiting needs to be induced within two hours of ingestion. Clinical signs usually occur within 6-12 hours post-ingestion. Mild signs can include increased thirst, restlessness, vomiting and diarrhea. More severe signs can include hyperactivity, increased urination, muscle tremors, unbalanced/uncoordinated, seizures, increased heart rate, increased breathing, coma and death.