What is Criteo ?
“Criteo enables e-commerce companies to re-engage with website visitors who have left their site via retargeted personalized display ads. These ads are dynamically generated on-the-fly with product recommendations based on the visitor’s product-viewing history.”
What does this mean to you ?
If you are browsing on a shared computer you may see adverts on sites you visit displaying a list of the websites and products the previous user has just visited, or worse still, the person who uses the computer after you may see the websites/products you have been viewing.
What is wrong with Criteo doing this ?
For a start Criteo could ruin your Christmas by showing you the gifts your spouse has been looking at, as what happened with me 
Worse than this is the privacy issues, do you really want the next person using your shared computer seeing the exact products (not just websites) that you have viewed?
Who uses Criteo in the UK ?
So far the biggest 2 merchants I have seen using Criteo to display their ads are Debenhams and John Lewis
Which websites display Criteo ads in the UK ?
The sites I mainly see Criteo ads on are Youtube, and shockingly, The Guardian.
I say shockingly The Guardian as I really thought they would have looked at the privacy issues of using Criteo