SEO is necessary for all websites. It allows to first be present on the search engines but also to be well placed for the use of keywords.
The most popular search engines are Google, Bing, Yahoo!, DAZOO, Ask, Altavista, Baidu (Chinese), LeMoteur …
We all know, the main one : Google, reporting 98% of web traffic on search engines.
SEO refers to many professional techniques to increase your visibility on Google and your ranking (Page Rank).