There are two main kinds of SEO, on-page search engine optimization, which includes keyword stuffing and streamlining site architecture and what I'll call off- page SEO, which focuses mainly on link building.
In the past, webmasters could rely on keyword stuffing, jamming their pages with an excess number of a specific keyword to get ranked in search engines for that term. This often resulted in unreadable content that nobody should ever be forced to sift through. Thankfully, this no longer is the dominating force in SEO -- though it still has its effects.
Now linkbuilding is the predominant means of artificially getting a site noticed. I use the term artificial to make a clear distinction from 'organic' links -- links that you would have gotten anyway because your content is actually 'link worthy'.
Nowadays, many inferior sites are ranking on page 1 for competitive keyword combinations because their owners are getting hundreds or thousands of links by spamming. Just recently, one of these spam sites, kosherhibachi.com, managed to nab the 2nd spot for the term 'auto insurance' before it was caught by Google and de-indexed.
Ultimately, this type of linkbuilding ruins the Internet because it guides users to complete and utter crap. I wouldn't have so much a problem with SEO if the pages that were benefitting from this process were actually worth visiting but that's almost never the case.
Getting noticed on the Internet needs to be a natural process that happens organically.
Coming Soon: Why SEO is necessary