Neuropsychologists deal with organic diseases of the brain and how these same diseases affect the psychological components of an individual such as their behaviour. As Paul and Ayesha point out, these diseases and syndromes are many and inlcude tumours and lesions as well as dementia.
These issues are crucial to understand - since in the arena of the courtroom they may serve as mitigiating factors and answer to the culpability of the individual in question.
In essence neuropsychologists bridge the gap between brain and mind, they understand both the hardware and the software which the brain gives birth to.
Mostly neuropsychologists deal with developmental disorders like autism, learning disorder( dyslexia) , attention disorder( ADHD). They also diagnose epilepsy, concussion, Stroke, dementia and etc.
Good questions. Generally they are answering legal questions which have some neuropsychological component. As a result, the most common diagnoses will be various neurocognitive disorders. Frequently they are considering the deficits caused by stroke, dementia, and/or head injuries. Other considerations might include epilepsy/seizures and delirium.