What is search-engine optimization?

People often say, “I want a number one ranking in Google!”.
It is actually not very difficult to be ranked number one. You could achieve this pretty quickly with an unpopular search term that can only be found on your website. The problem is that no-one searches for those kind of terms.
Search-engine optimization, or SEO for short, is the process of ensuring that you are ranked highly in the search results for terms that are frequently used and highly relevant for your field.
Which keywords are relevant for me?
First of all, it is important to understand that a search term may consist of one or more words. So for example, you might search for the term “children’s shoes”.
If you were the owner of a children’s shoe store in, say, San Francisco, it is unlikely that this search term would be especially relevant for you unless you had an online store and shipped nationwide, or even worldwide.
Instead, it would be more relevant to rank highly for the search term “children’s shoes in San Francisco”, or even “children’s shoes in Mission District, San Francisco”.
You will have to find out for yourself which search terms your customers are using. Google Keyword Tool can give you some ideas to start with, and this blog post contains a video with an excellent overview of the topic of keyword research (it uses a fee-based tool but you can just as easily carry out your research using the free Google Keyword Tool).
How search engines work
Search engines like Google look at different things to assess your website. The content of the website itself, known as “on-page optimization”, is important. It also looks at how other websites rank your site, called “off-page optimization”. The number of links to your website is a factor here so the more links you can get which are relevant to your area, the better. These links contribute to a steadily increasing Pagerank for your website.
As soon as Google starts to find links to your website, you will start to appear in their search results. You can also manually register your website with the search engines.
Read more about on-page optimization.
More information
Google’s own starter guide to SEO
Also recommended: the SEOmoz Beginners Guide