SEO or Search Engine Optimisation is the name given to activity that attempts to improve search engine rankings.In search results Google displays links to pages it considers relevant and authoritative. Authority is mostly measured by analysing the number and quality of links from other web pages.
Search engine optimization is a methodology of strategies, techniques and tactics used to increase the amount of visitors to a website by obtaining a high-ranking placement in the search results page of a search engine (SERP) -- including Google, Bing, Yahoo and other search engines.
So How to SEO a website on Google, Bing, Yahoo and other search engines? in this topic i will present step by step all techniques to SEO a website but the first time you need to understand some basic terms bellow:

Keywords
SEO keywords are the key words and phrases in your web content that make it possible for people to find your site via search engines. A website that is well optimized for search engines "speaks the same language" as its potential visitor base with keywords for SEO that help connect searchers to your site.

Keyword Research
Keyword research is one of the most important, valuable, and high return activities in the search marketing field. Ranking for the right keywords can make or break your website. By researching your market's keyword demand, you can not only learn which terms and phrases to target with SEO, but also learn more about your customers as a whole.

Keyword Density
Keyword density is the percentage of times a keyword or phrase appears on a web page compared to the total number of words on the page. In the context of search engine optimization keyword density can be used as a factor in determining whether a web page is relevant to a specified keyword or keyword phrase For example, you have 1000 words article, then having a 1% keyword ratio means that you should have the presence of your keyword one time within that 100 word text.

On-Site SEO
In search engine optimization, on-page optimization refers to factors that have an effect on your Web site or Web page listing in natural search results. These factors are controlled by you or by coding on your page. Examples of on-page optimization include actual HTML code, meta tags, keyword placement and keyword density. On-site optimisation is the foundation on which the off-site optimisation process builds upon.Without it, off-site optimisation efforts can go largely to waste

Off-Site SEO
Off Site Search Engine Optimization is accomplished from sources other than your site. This is achieved “off the pages” of your website to increase search engine performance for target keywords that are related to your “on page” content and the keywords that exist in off site direct links.Off site SEO is link building,Link building is probably one of the most talked about (and debated about) SEO tasks. The basic goal of link building is to get other websites to link to yours. If you think about ranking at the top of search results as a popularity contest, then links are like votes saying your website deserves to be ranked highly. Getting links with keyword anchor text will help you rank specifically for the keywords that are linked to.

In this Category i will show detail some simples ways to SEO a website including:
 How to SEO A Website
Tools And How To Use to SEO