You have a Google account and search for something and suddenly find that everything you search for suddenly appears when you search.
For example I searched for a video camera then that search showed up in everything I did.
These are called Interest based ads , which are based on your interests, previous visits to other websites, or demographic details.
Well you can opt out of these.
The following link will give you the instructions however the more that Google knows about you the better the search experience.