What Do You Consider As White Hat SEO?

Im wondering, what's white hat SEO to be exact? Directory submissions, article submissions, and that's it? Because paid links obviously aren't white hat and even 3 ways or 4 ways link exchange aren't part of white hat SEO either (because they meant to cheat). So what are part of white hat SEO? I heard a lot of getting relevant backlinks, but how to get relevant backlinks in white hat way other than paying for the links or do 3-4 ways link exchange?