__utmb
This is one of the four main cookies set by the Google Analytics service which enables website owners to track visitor behaviour and measure site performance. This cookie determines new sessions and visits and expires after 30 minutes. The cookie is updated every time data is sent to Google Analytics. Any activity by a user within the 30 minute life span will count as a single visit, even if the user leaves and then returns to the site. A return after 30 minutes will count as a new visit by a returning visitor.
__utmc
This is one of the four main cookies set by the Google Analytics service which enables website owners to track visitor behaviour and measure site performance. It is not used in most sites but is set to enable interoperability with the older version of Google Analytics code known as Urchin. In the older version, this was used in combination with the __utmb cookie to identify new sessions/visits for returning visitors. When used by Google Analytics, this is always a Session cookie which is destroyed when the user closes their browser. The main purpose of this cookie is: Performance
__utmz
This is one of the four main cookies set by the Google Analytics service which enables website owners to track visitor behaviour measure of site performance. This cookie identifies the source of traffic to the site - so Google Analytics can tell site owners where visitors came from when arriving on the site. The cookie has a life span of 6 months and is updated every time data is sent to Google Analytics.