It's called 'remarketing' and is amazingly easy to do .. if you know how.
Go to a commercial site once, and you'll see the ads pop up on other random sites.
The concept is simple; you showed interest in the product, so nudging you occasionally is not out of the question.

They don't care what other sites you visit, or look at your browsing history or 'track you'. Like I said it's pretty simple.

You come to the site. You get a 'cookie'. This cookie lets the ad provider know you've been to the site, so a related ad is a 'preferable' ad, as opposed to a 'random' ad. Thus you get the related ad. It feels like tracking, but it has a line in the sand when done by the rules.

Clear your cookies or browsing history and it will go away (if done ethically).

I can show you the code that does it, and show you the actual 'cookie' if you want.
The cookie can be set to expire in anywhere from 30 to 120 days usually. Depending on the market, the expiration is up to the programmer/marketing team of the site you visited. If you don't want them either turn them off or don't visit sites that use them!

/Europe has different rules than US on the use of cookies, so if you are in US it will likely be more prevalent. /

Trust me; they don't care what other sites you visit, but if you show interest in their product it seems logical to use the ad systems to remind you and possibly prompt a sale. It's really nothing more than that.

They don't know 'who' you are, or what other sites you visited. They simply know you came to their site, got a cookie, and since they pay for ads, their ad gets displayed with preferential priority. It's really a win/win if you think about it. You get ads related to things you're interested in, and they get more focused ad placement.
Like I said, they aren't really 'tracking you' like some people think.

Edit:
The 'how it works' side is pretty simple, but running a campaign with good ROI can be pretty involved from the other side. Knowing which sites to pay for and excluding others, controlling the budget (etc.) takes a lot of thought/effort. I certainly didn't mean to downplay the work of a good retargeting/remarketing campaign. I was simply commenting on the technology and use in my above post .. you are not being 'tracked' in the sense some people think they are.
Yes, Google tracks you, since they offer the AdSense program, but again not the way some people assume.

Last edited by rharv; 08/08/15 09:54 AM.
I do not work here, but the benefits are still awesome
Make your sound your own!