I notice a lot of websites I visit are flooded with pop-ups upon entering. Thankfully I have a pop-up blocker on. I was just curious as to why that happens. I don't want that to be an issue with my site.

Many of the popups display advertisments, which is a method the web sites make money. I find them really annoying and usually ignore them. There are better ways to make money than send many popups to a visitor.

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You and the majority of web users. If a site throws me a pop-up I'll exit the site as quick as I can and not by clicking on the ads. It worse when you get a high profile site using pop-ups, you would think they would know better on how annoying they are.

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Online Backups Ireland & Data Protection

So, when I start my website, this won't happen unless I agree to take on the advertiser who makes these???

That is correct. Its the choice of the site owner if they want to use pop-ups or not.

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Online Backups Ireland & Data Protection