To most of us, chocolate is a delicious substance. To dogs, chocolate is also delicious, but potentially lethal. Chocolate contains theobromine, a naturally occurring stimulant found in the cocoa bean. It affects the central nervous system as well as heart muscle.
It's the theobromine that is poisonous to dogs in sufficient quantities. But it takes, on average, a fairly large amount of theobromine to cause a toxic reaction.