Parkinson's disease belongs to a group of conditions called movement disorders. The four main symptoms are tremor, or trembling in hands, arms, legs, jaw, or head; rigidity, or stiffness of the limbs and trunk; bradykinesia, or slowness of movement; and postural instability, or impaired balance. These symptoms usually begin gradually and worsen with time.  As they become more pronounced, patients may have difficulty walking, talking, or completing other simple tasks.  Not everyone with one or more of these symptoms has PD, as the symptoms sometimes appear in other diseases as well.

PD is both chronic, meaning it persists over a long period of time, and progressive, meaning its symptoms grow worse over time. It is not contagious. Although some PD cases appear to be hereditary, and a few can be traced to specific genetic mutations, most cases are sporadic — that is, the disease does not seem to run in families.  Many researchers now believe that PD results from a combination of genetic susceptibility and exposure to one or more environmental factors that trigger the disease. 

PD is the most common form of parkinsonism, the name for a group of disorders with similar features and symptoms.   PD is also called primary parkinsonism or idiopathic PD. The term idiopathic means a disorder for which no cause has yet been found. While most forms of parkinsonism are idiopathic, there are some cases where the cause is known or suspected or where the symptoms result from another disorder.  For example, parkinsonism may result from changes in the brain's blood vessels.