What causes hand tremors?

Many. To mention few, parkinsonism, essential tremors, days tonic tremors, toxic tremors, psychogenic tremors etc etc.
Many causes. Hyperthyroidism, familial essential tremors, parkinson's disease, medications (e.g., depakote), too much caffeine, amphetamines, cerebellar problems (multiple sclerosis). I have even seen a local tremor associated with peripheral nerve weakness in an arm. There are treatments for all of the above.