What is the difference between parkinsonism and Parkinson's disease?

Parkinsonism is a syndrome diagnosed on the presence of:

• A resting tremor - usually in one limb (esp. one hand), and it disappears with voluntary movement.
  
• Rigidity - a term that is too non-specific for my liking, but which here refers to a resistance to passive movement of the joints
• Bradykinesia - slowness and paucity of movement
• Postural instability - an unsteadiness on their feet, whether standing or walking

Not all these features need to be present; postural instability, for instance, generally only occurs late in the disease course (and is non-specific anyway).

There are numerous causes, such as certain medications, encephalitis and toxins.  However, the commonest cause is:

Parkinson's disease, which is a specific entity caused by a loss of dopamine-secreting neurones in the basal ganglia, specifically in a part of the brain called the substantia nigra, and which is deemed to be primary, or idiopathic.  In other words, no specific cause of the loss of dopaminergic neurones can be established in Parkinson's disease (although theories abound).  This latter stipulation is important, since many of the other causes of parkinsonism also cause a loss of dopamine-secreting neurones in the substantia nigra.