Tourette syndrome (TS or Tourette's) is a common neuro-psychiatric disorder which usually starts in childhood and is characterized by multiple motor tics and at least one vocal tic. These tics characteristically wax and wane in intensity and frequency, can be suppressed temporarily, and are typically preceded by an unwanted urge or sensation in the affected muscles. Some common tics are eye blinking, coughing, throat clearing, sniffing, swearing, and facial movements.

It's known that the great majority of cases of Tourette syndrome are inherited, but neither the mode of inheritance, nor the neurological mechanisms are known. No linked gene has been identified.

Whilst not directly causing TS, environmental, post-infectious, or psychosocial factors can influence its severity.

The exact mechanism affecting the inherited vulnerability has not been established, and the precise cause of Tourette syndrome is not known. Tics are believed to result from dysfunction in the central nervous system, in the cortical and subcortical regions, the thalamus, basal ganglia, and frontal cortex of the brain. Neuroanatomic models implicate failures in circuits connecting the brain's cortex and subcortex, and imaging techniques implicate the basal ganglia and frontal cortex."

Source : Wikipedia

Further info from the Tourette Association of America.

The causes of Tourette and other Tic Disorders remain unknown. The conditions are hereditary and so genetics clearly play a role in many, if not most or all, occurrences of the conditions. Environmental, developmental or other factors may also contribute to these disorders but, at present, no specific agent or event has been identified. Therefore, TS and Tic Disorders are likely to be caused by complex interactions between genetic and other factors which may vary in different individuals. Studies are underway to find the genes and other factors underlying the development of these disorders."