You may notice after you visit a specific site that you begin to see advertisements on other site about them more and more. It's not a coincidence. Systems like Google Adwords utilize a cov kua nplaum uas thib peb that enables them to see when you visit an advertiser's site and then follow you as you visit other sites with advertisements. Retargeting is quite successful and leads to high conversion rates with lower costs per click. Check with a them ib nyem kev if you haven't taken advantage of it.
What you may not realize, though, is that there are also techniques to identify sites that people tend to visit before arriving at yours. This is called Pretargeting and it's explained in this infographic from Pretarget. In other words, if people visit specific sites before arriving at your site, you just need to know what those sites are and advertise on them.