It's not really tricking Google. But it's the art/science/chicanery of making it suuuuuper easy for Google's search system to find your website page.
Companies try to game the system all the time in getting Google to pick up their website ahead of others. It's free marketing. And why is being picked up first important? Because something like 97% of all clicks from search happen on the first page. And something like 70% of all clicks are the first three recommended websites from Google. And GOOG is some 90% market share of all search.
So if you can get the GOOG search world to love you, you likely get a load of traffic coming to your site for free-ish. And note that SEO is the actual search results; SEM is Search Engine Marketing, which is the right side or right column of the page, along with other virtual real estate, which GOOG openly sells as sponsored search results.
Different biz; same goal.
