The number one difference between organic search engine placement and pay per click advertising is that, with pay per click advertising, as soon as you can't or won't outspend the next website you're off of page one. Unless you've invested in organic SEO as well. Here are some other major differences:
You pay for every click on your ad even if the user has no intention of buying your product or service. I have seen the cost per click exceed $50 for some searches.
Clicks are free for any search. Users can go to your website as many times as they need to to gather information and make an informed buying decision without increasing your customer acquisition costs.
When your accounts funds are depleted your ads are no longer displayed.
Your website's ranking is active 24 hours per day.
If you want to continue generating site traffic through search engine impressions, just keep purchasing ads every day.
If you want to continue generating site traffic through search engine impressions, just keep your site updated with information relevant to your business, product, service, or industry.
You have to outbid your competitors for Ad spaces.
You have to provide better information than your competitors.