Mostly, essential tremor is a condition that's poorly understood but results in tremor without the other problems that are associated with Parkinson's. The basis for essential tremor is not well understood. There is currently some controversy as to whether it may be associated with pathological changes in the brain, including, to a milder degree, some similar to those seen in Parkinson's. Although it can be a very difficult and embarrassing disorder, I would say it's a more benign disorder.
You're well aware of history, that Prime Minister Diefenbaker had essential tremor, and Katharine Hepburn, but it doesn't produce the slowness, poverty of movement, and the other problems associated with Parkinson's.