Many web analytic softwares use referrer strings to determine whether a user is coming from a search engine, other websites or directly to your website.
Many users also have custom scripts that show up different ads or messages to search engine users, if you have been using such customized scripts, Google.com search referrer URLs are set to change starting next week.
The major change in the referrer link would be the way Google URL is constructed, so instead of seeing:
Users will now see a new URL which looks something similar to this:
The major difference in the referrer URL is the change from /search to /url, if you have been using this in your custom scripts, make sure to start handling the new referral URL structure.
You can read more about this change and how it will affect you at the Official Google Analytics Blog.