Ataxia is a neurologic syndrome characterized by clumsy and uncoordinated movement of the limbs, trunk, and cranial muscles. It results from pathology in the cerebellum and its connections, or in the proprioceptive sensory pathways. The list of causes of ataxia is extensive. Typically, ataxia is classified on the basis of whether it is hereditary or acquired. Ataxia may also be classified by age of onset (childhood versus adulthood) or by whether it is associated with other clinical features (e.g., seizures, dystonia, vision loss). Degenerative ataxia is the term used to denote ataxia related to cerebellar atrophy of both genetic and unknown causation. Other terms that have been used for this type of ataxia in a broad sense include spinocerebellar degenerations and olivopontocerebellar atrophy, but these are becoming obsolete. http://www.ataxia.org/ [National Ataxia Foundation]
Cerebellar ataxia produces a characteristic set of signs.  Gait becomes unsteady with tendency to falls, hand coordination is impaired, and the patient may experience dysarthria or dysphagia. There may also be ocular symptoms related to abnormal control of eye movements. Neurologic examination shows dysmetria of limbs (inaccuracy of targeted movements) as shown by abnormal finger-chase using patient fingers or toes. There is kinetic tremor of limbs and an uncontrolled oscillation of limbs during relatively slow but targeted movements. Early stance and gait problems include the inability to do a tandem stance or stand with feet together; stance becomes broad-based and displays increased sway of the body. Tandem gait becomes impaired and, later, regular gait can be frankly ataxic with a broad-based and lurching quality. Eye movements show gaze-evoked or other types of nystagmus, abnormal pursuit of visually presented objects (jerky appearance due to intrusion of saccades into pursuit), and inaccurate saccades when the person is asked to move the eyes quickly toward a target (hypometric or hypermetric saccades). There is also a scanning type of dysarthria.
Sensory ataxia is characterized by lower-limb and upper-limb incoordination associated with lack of proprioception. Clinical signs include impaired vibration sense, as well as impaired position and kinesthetic sense. Deep tendon reflexes are often absent but eye movements and speech are not affected. In some patients, sensory and cerebellar features can coexist.
Patients with ataxia as a major feature often exhibit additional noncerebellar neurologic signs, either because the lesion causing the ataxia extends beyond the cerebellum into neighboring structures or because the degenerative condition involves structures beyond the cerebellum.
There are no precise data regarding the prevalence of ataxia of all causes.   Epidemiologic studies that focus on hereditary types of ataxia have shown a prevalence of around 10 per 100,000 population, and idiopathic ataxia probably outnumbers hereditary cases.  Since there are many causes of degenerative and inherited ataxias, each individual type of ataxia is relatively rare. In the case of genetic forms of ataxia, there are clusters of high incidence of specific types due to founder effects and ethnic and geographic variations in the prevalence of many mutations. 
Ataxia may be acquired or genetic. The list of acquired causes is extensive and includes vascular, demyelinating, neoplastic, autoimmune, toxic, degenerative, compressive, and infectious etiologies. In many of these, ataxia may be a major clinical finding or be overshadowed by other signs. Many acquired lesions can be easily identified on imaging studies. However, imaging studies may reveal only cerebellar atrophy, a nonspecific finding, necessitating further diagnostic workup.
Genetic ataxias may be further classified by their mode of inheritance, such as autosomal dominant, autosomal recessive, or X-linked. In many of the commonly identified genetic ataxias, the clinical picture is dominated by cerebellar signs. However, many inherited metabolic errors, especially in childhood, can have ataxia as a major or minor feature. http://www.ncbi.nlm.nih.gov/omim/ [Online Mendelian Inheritance in Man (OMIM)] In general, genetic forms of ataxia result in cerebellar atrophy and atrophy of related structures, which can be seen on imaging studies.
The inherited ataxias have recently been the focus of increasing understanding of molecular mechanisms.     More than 40 distinct gene defects cause ataxia as a major feature, and recessive, dominant, X-linked, and mitochondrial mutations have been identified. A wide range of primary pathogenic mechanisms have been proposed, including oxidative stress, problems with respiratory chain function, cytoskeletal abnormalities, DNA repair abnormalities, chaperone protein dysfunction, protein misfolding and aggregation, and channel dysfunction, among others.
Many types of gait problems can superficially resemble ataxia. Examples include gait abnormalities from bilateral frontal lobe lesions (i.e., frontal ataxia or Brun ataxia) and occasionally dystonic gait. Gait disorders from muscle disease, neuropathies, and spinal cord problems can usually be easily differentiated by clinical exam.
Gait difficulties from lesions of the vestibular apparatus also need to be differentiated from cerebellar ataxia. The coordination problem in the limbs is in the nature of "past pointing" rather than true dysmetria, and eye movement problems often are confined to nystagmus in the primary position. Dysarthria is not a feature of vestibular disease, but nausea and oscillopsia can be a feature of vestibular lesions.
The presence of speech and oculomotor signs related to cerebellar pathology can clearly differentiate cerebellar ataxia from other types of incoordination.