Tetanic contractions and tremor are different phenomena.
Tetanic contraction (or tetanus) refers to sustained contraction of a muscle subjected to high-frequency stimulation. With this high frequency, a new contraction begins before the previous one has ended. Thus the contractions get 'summed up'. You could see this interesting video for a demonstration.
Tremor is an involuntary shivering movement which is caused by alternating contractions of opposing muscle groups. The causes are many, ranging from physiologic tremor to degenerative disorders of the nervous system (such as Parkinson disease).
Note that tetanic contractions are not a symptom of Parkinson disease. In fact they are not a symptom of anything at all, inasmuch as they are a physiological property of isolated muscle. Tremor, on the other hand, is a phenomenon seen in muscle groups, and is one of the core symptoms of Parkinson disease.