The CHADS2 score is a clinical prediction tool that estimates the risk of a stroke in patients with atrial fibrillation (non-rheuumatic). It is used to determine whether the patient should be treated with anticoagulation or antiplatelet therapy in order to reduce their risk of stroke. There is also an updated version (not yet used by NICE, but used elsewhere in Europe) called the CHA2DS2-VASc score that is able to better startify low risk patients (see menu to left for link to this clinical calculator). In the UK, NICE (National Institute for Health and Clinical Excellence) recommends the use of the CHADS2 scoring system.
You must give information about all five criteria - congestive heart failure, hypertension, age, diabetes and stroke/TIA - in order to startify the patient for the risk of stroke.