Third party tags are snippets of code provided by third party vendors or partners which are implemented on a website and which generate browser calls to external third party servers.
Third party tags are very common and serves many purposes on a page or website:
ad serving (see third party ad tag)
widget integration (embedded videos, etc..)
social media integration
conversion tracking (email, PPCSE, display, analytics)
remarketing / retargeting
A/B testing and MVT
voice of the customer (surveys and feedback)
A 2012 E-consultancy study found an average of 14 third party tags per website and according to research from the University of California at Berkeley Center for Law and Technology, 2324 third-party cookies were counted on the home pages of the most-popular 100 websites in October 2012.
Some websites are known for having dozens of third party tags.
Third party tagging evolution (image credit IAB’s "Site tagging best practices"):
Some issues are associated whith third party tags proliferation:
increase of page loading time (bad for visitors and search engine ranking)
privacy issues ( visitors are usually not really aware of sharing data with other actors than the visited website)
IT team sollicitations for implementation
potential data leakage
Third party tags multiplication and related issues have led to tag management solutions.
Below Ghostery tools detects 8 third party tags on CNN home page:
Ghostery, a browser plugin which detects third party tags, lists and identify nearly one thousands tags:
A well designed video showing third party tagging issues: