How to launch an ad on a visitor who visited sites about water heaters?

Question

contextual advertising

How to launch an ad on a visitor who visited sites about water heaters?
For example, I recently visited the page of a site with a water heater at work, later I went to classmates and I started to see ads with water heaters from another site.
What is this type of advertising called?
I know that this can be set up in Google adwords on CCM sites, but how?
I realized that in classmates you can, but how is the question too?
In what networks and social networks you can set up such advertising, please enlighten me.
Thank you.

Answer 1

Rsya Direct, Maitarget, advertising cabinets VKontakte and Facebook. KMS quality of sites in .ru is questionable.

Answer 2

How to launch an ad on a visitor who visited sites about water heaters?

This is called retargeting or remarketing. There are both in contextual advertising (Ya. Direct, Google Adwords) and in social networks (VKontakte, Facebook, classmates).
Do not confuse it simply with showing ads in YAN / GMS, which is shown to you on thematic sites, or after you have entered a search query, but did not go to a specific site.