may be what you mean is: search engine optimization and natural search engine optimization. natural/organic search engine optimization is a process of trying to get your site ranked better in search engine results. meaning your site "naturally" appears at the top ranked position at a search engine result page. of course you can make your site appear at top ranked position at a search engine result by buying the search engine PPC ads. eg. google PPC ads, a.k.a. AdWords, or Yahoo Search Marketing ads, or MSN ads.
is there such thing as 'natural searh engine'? if there is, maybe there is an 'unnatural search engine' then.