The top 3-4 spots on a Google can be bought as 'ads'. These ads fall under the category 'PPC', or pay-per-click. If you want one of those spots, you pay for the 'keywords' that will get you there. They are usually very expensive, but all it takes is a call to Google to find out how much it will cost you.
SEO is 'search engine optimization', and has more to do with your site being found in searches in an 'organic' manner. Organic would be all searches NOT paid for in ppc. Everything that falls under those paid searches on Google (they are also on the right side of the results page), are 'ads' and not organic. There are sites that can help you rank higher in organic searches.
What's the difference between the two? Ppc gets you consistent results but costs a lot. Many consumers won't look at paid ads on search results b/c they know they're ADS. So they look below, where the organic results start, believing (maybe) that they are more legitimate results and got there from 'natural', organic ranking.
Hope that wasn't too confusing. It's my 2 cents from experience.
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