From http://parkinsonsaction.org/about-pan/parkinson...
This article gives a good summary of the difference.

Parkinsonism is the clinical definition of a variety of different underlying pathologies that can cause Parkinson’s-like symptoms such as slowing of movement, tremor, rigidity or stiffness, and balance problems. There are a number of disorders that can produce the symptoms referred to as Parkinsonisms; Parkinson’s disease is just one of them.

So, what’s the difference between being diagnosed with Parkinson’s disease and having what are known as Parkinsonisms? According to Dr. Dickson the typical Parkinson’s patient has Lewy bodies (aggregates of a protein called alpha-synuclein) in the brain’s neurons. When that patient is given dopamine replacement therapy (e.g., Sinemet), those symptoms go away.

By contrast, Parkinsonisms – also sometimes referred to as atypical Parkinson’s –have features in addition to typical Parkinson’s disease symptoms, and those symptoms do not respond to dopamine replacement therapy.

I was diagnosed with Parkinsonism 2 years ago. At that time I was a mess. Urs all the symptoms. Including even hallucinations. I was diagnosed drug induced parkensonism from compizine. It was hard getting the meds right. And in many cases of drug induced cases stopping the meds may resolve the problem. In my case not so much. Due to the length of time on med. After 2?years I can move, swallow, many other things I’m much better if you seen you before, and after you would not believe I am the same person. However, taking the Carbidopa has it issues to so you have to pick the best of two evils. My doctor has also help me with palliative care that includes such things as testosterone injections. I have chosen quality of life over quantity of life. I believe I made the right choice.

Some good articles about msa http://www.ninds.nih.gov/disorders/msa/detail_m...
https://rarediseases.org/rare-diseases/multiple...

Treatment:
here is no specific treatment for MSA. Treatment is aimed at controlling the symptoms of the disease. Drugs that are used to treat people with Parkinson’s disease, most notably levodopa (given in tablets of Sinemet), may also be prescribed for individuals with MSA. However, the effectiveness of such medications varies greatly among affected individuals. In many cases, individuals do not respond or respond poorly to such therapy. Approximately 1/3 of affected individuals respond to levodopa therapy. However, in most cases, the effectiveness of this therapy decreases over time. In addition, these drugs must be used with caution because they may lower blood pressure.

What I have is atypical parkinsonism specifically MSA-c, formally called shy-Drager syndrome. Parkinsonism is an umbrella term which includes idiopathic Parkinson disease, Dementia with Lewey Bodies, PSP, MSA-p, MSA-c. They are all under one umbrella but with similarities and differences.

I have Parkinsonism and am taking Sinemet. It does help but does not completely eliminate the tremors. Dosage has been increased but tremors continue. Probably on my next visit to neurologist they will want to increase dose again. I don't know if I want that because I worry about side effects.