I use Firefox YSlow plugin to check web site performance. It is quite good tool for developer.
The cookies named _utma , _utmb, _utmc and _utmz come from those websites that using Google Analytics as site analytic or tracking. The main purpose of it is to track web site user visit activities.
The cookies names likely come from the very early version called the Urchin Tracking, and now it is ga.js.
- __utmz stores where a visitor came from (search engine, search keyword, link), referral .
- __utma stores each user’s amount of visits, and the time of the first visit, the previous visit, and the current visit (presumably partly for double checking of this information).
- __utmb and __utmc are used to check approximately how long you stay on a site: when a visit starts, and approximately ends (c expires quickly).
Refer to Google Help.