Generalised anxiety disorder is a persistent and common disorder, in which the patient has unfocused worry and anxiety that is not connected to recent stressful events, although it can be aggravated by certain situations. This disorder is twice as common in women than it is in men. Generalised anxiety disorder is characterised by feelings of threat, restlessness, irritability, sleep disturbance, and tension, and symptoms such as palpitations, dry mouth, and sweating.

The Lancet Volume 368, issue 9553, pp 2156-2166

The causes of GAD are unknown, though it's strongly associated with general levels of stress, along with environmental factors. It may also have a hereditary component, as it's known that close relatives of GAD sufferers are more likely to suffer themselves - but it has not yet been possible to determine whether this link is neurological, or because of common social and lifestyle factors.

It's estimated to affect around 5% of the general population.

Further information Wikipedia