I am a little new to the scene of SEO and advertising, but have some decent knowledge of web development/coding. Anyways this is some thing always made me wonder whats its called and how its done ( I have a few ideas already.) But sometimes when I visit websites no matter where I click on the website for the first time I get redirect to a popup/ad. How are they overlaying the website with a clickable ad, like this. Thanks for any replies or help.