In regards to the FDA guidelines posted by la82, I tend to personally agree with their overall classification of devices into SR and NSR classes. In addition, I would like to highlight that the decision boards pay special attention to HOW the device is proposed to be used in addition to the actual device. The intended use of a device could change the determination from NSR to SR depending on any potential of lasting effects of a NSR device. For example, standard daily contact lenses are a NSR device as they pose no direct risk to the body. However, long term, overnight contact lenses are classified as SR devices because of this intended use poses potential for serious harm to the subject.
http://www.fda.gov/downloads/RegulatoryInformation/Guidances/UCM126418.pdf
http://www.eandireview.com/index.php?option=com_content&view=article&id=195&Itemid=149