What are these cookies called, and what do they do?

_utma  tracks the number of times a visitor has been to the site, when their first visit was, and when their last visit occurred. It expires after 2 years. _utmb  and  _utmc  track how long a particular visit takes.   _utmb  expires after 30 minutes, _utmc  expires once you close your browser. _utmz  tracks which site or search engine the visitor came from, the link clicked or keyword used, and the visitor’s geographic location. It expires after 6 months. These four cookies are placed by  Google Analytics and help us understand how visitors interact with our website. Google Analytics does not collect personally identifiable  information, and we do not share our collected data with Google.  Read more about Google Analytics  or download the Google Analytics Opt-out Browser Add-on.