Essential tremor (ET, also referred to as benign tremor, familial tremor, or idiopathic tremor) is a progressive neurological disorder that is also the most common movement disorder. The cause of the condition is currently unknown. Source: Wikipedia
Unlike Parkinson's disease the tremors occur when the patient is trying to perform a movement, rather than when at rest.
In about half of cases, the disease has been associated with genetic abnormalities, though no genes have yet been identified. It's presumed environmental factors may also contribute.