SEO is the abbreviation of Search Engine Optimization, which means the principles and rules that can be applied to the web site to optimize the way it's accessed by the search engines like Google, Yahoo or MSN.

Why we need it? - The more content from your web site is crawled and indexed by a search engine the more users can find it on the web.

What means crawling and indexing? - To understand this let's speak about search engines and the principles they work.
One can think that Google goes ahead and searches the internet when the user enters keywords she's interested in, but it's not true. Otherwise the searching process would take several hours instead of the resulting second.
Actually Google searches its own database, where the information about the world-wide-web is collected. At this moment you can understand what means indexing - a web site can be said is indexed when the information about it is contained in the search engine database, in this case Google's database.
Now let's talk about crawling. Crawling is the process of getting the content of a website from the internet. As the internet is a very large network that becomes larger day by day, crawling is not easy (actually it's impossible) to be done by hand, that's why this process is done by the computer software called Spider. I'll try to cover the principles of developing spider later.
Some people often make mistake saying that the crawling and indexing is the same process. Actually crawling takes the preceding place. Moreover, if a website has been crawled it isn't the warranty of being indexed. The step that can be taken between crawling and indexing varies from one search engine to another, in most cases it's the administrative and/or security step, such as approving the content to become indexed.

As you understand SEO is useful step that is recommended to be taken as long as you are interested to attract people to your web site.

We have already answered the question in the title of this post. Later I will cover the steps of SEO that you can follow to achieve the success in web publishing and web authoring.