There are two types of SEO which are practiced
Black Hat SEO - Black hat SEO refers to a set of practices that are used to increases a site or page's rank in search engines through means that violate the search engines' terms of service. Uses stuffed keywords, duplicate content, redirecting users to other sites, and links from sites with non-relevant content. Black Hat SEO tactics can get your website banned from Google and other search engines.
White Hat SEO - SEO tactics that are in line with the terms and conditions of the major search engines, including Google. Has relevant content, proper keywords, well-labeled images, proper links.