Retargeting / Remarketing
Retargeting / remarketing is the technical name for those adverts that you see around the internet after visiting a website. Some companies use it to remain at the front of your mind after you visit them on the internet.
It is a strategy that can help to increase sales if you normally sell online. However, you need to consider whether your customers are likely to be irritated or "freaked out" by the fact that your adverts follow them everywhere you go.