Google Analytics uses cookies to track visitors and find out useful information about them. Information such as average time on site, pages viewed per visit and whether a visitor is new or returning are just some examples. Google Analytics sets 4 (officially 5) cookies when you land on a website (with its tracking installed), these are:

__utma

Essentially this cookie tracks:

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  • How many times a visitor has visited the site
  • The first time a visitor found your site.
  • A visitor’s last visit.

This type of cookie does not have an expiry date and is known as a ‘persistent cookie’ – Technically all cookies must have an expiry date, but the expiry date for this cookie is set to a few years, and as the cookies are reset every time a user arrives at the site it is safe to say that it has no expiry date.

__utmb &__utmc

These two cookies work in unison to calculate time spent on site. __utmb notes when the visitor arrives on the site and __utmc stores when the visitor leaves.
__utmz

This cookie stores information about how a visitor arrived on your site. Did they type in the URL directly into the browser? Have they come from an affiliate site? Or Did they come from via a search engine? The cookie stores a range of information in different variable, they are:

  • The campaign source (utmcsr) – e.g. Search engines like :Google, Yahoo, MSN, or another source like an affiliate site.
  • The campaign medium (utmcmd) – e.g. If we take for example a visitor arriving via a search engine – the campaign medium stores whether they came via an organic search result or paid search ad. If a user arrives directly this variable will be set to (direct) as well as the campaign source variable. The campaign medium mainly applies to search engines so that you can differentiate between paid/organic traffic but it can also be set for other uses – an example is if you have two links on an affiliate site then you may want to differentiate between visitors from each link and so you could set the variable accordingly. 
  • The campaign term (utmctr) - or keyword/search term. E.g. If I arrived via a search engine by searching for ‘will martinez’s blog’ then: utmctr = will martinez’s blog
  • The campaign content (utmcct) - used for A/B testing in paid search campaigns as you can pass an ads content straight into the cookie’s variable.
  • The campaign name (utmccn) - E.g. In Google Adwords the name of your ad campaign.
    utmgclid unique identifier used when AdWords auto tagging is enabled

__utmv 

If you are using the user-defined field for Google Analytics reporting then this cookie will be set with that value.