Search Engines require that they be submitted with a search word/phrase (keywords) which are related to the information you are searching for.
Once they get the keywords they search their databases to match the keywords with web sites which contain related content, and then spew out an ordered listing of those web sites based upon how they rank them with respect to the search term.
By optimizing your site for search engines you try to design your site such that the search engines relate your site favorably with your targeted search term.
The internet is full of sites promoting site optimization techniques that actually make your site less usable to your end users, although they might seem to make them more attractive to search engines for the targeted search terms. However, one should stay away from such techniques, simply because anyone who has done business online or offline will vouch for the fact that it is more difficult to acquire new customers rather than retain existing customers. A site with a bad usability will just not be able to retain its existing customers, and if a site can't retain its existing customers, it is little use spending time and money for it to acquire new customers. The best search engine site-optimization techniques are those that enhance your sites usability for your customers as well as for search engines.
The first step of site-optimization for search engines is to narrow down on your targeted keywords or phrases. For example the target audience for this SEO tutorial will mostly use the search terms 'SEO' or 'Search Engine Optimization' or 'SEO Tutorial' in their favorite search engines.
The thing to note here is that some of these search terms will be targeted by a relatively huge number of web sites Hence if one targets those very search terms it will very difficult to reach high rankings simply because of the very high competition involved. Prudence lies in targeting relatively less popular search terms.
One way to find out highly popular search terms is by using the Google Suggest tool.When one types search terms in Google Suggest it shows the number of times that term has been searched for in the past. Additionally it also shows some other popular and related search terms. For example, on typing 'SEO' in Google Suggest we get the following results:
Figure 1: Google Suggest tool results for 'SEO'
This shows that the keyword 'seo' has been searched for some 15,300,000 times. However, if we keep typing further and check out 'seo tutorial', this is what we see:
Figure 2: Google Suggest tool results for 'seo tutorial'
The search term 'seo tutorial' is less highly searched with a figure of only 962,000. What this means is that often it will be easier to get listed in the top 10 results on major search engines if you target your site for the keyword 'seo tutorial' rather than targeting for the keyword 'seo'.
One other interpretation of the above figures is that it is more lucrative to list in the top 10 for the keyword 'seo' as compared to the keyword 'seo tutorial', simply because a top 10 listing for 'seo' will drive higher amount of traffic to your site as opposed to 'seo tutorial'. However, this is easier said than done. Getting your site listed in the top 10 results for a super competitive term like 'seo' could take months (at least in Google), even if it gets listed in the first place.
More often than not your choices boil down to either getting a poor ranking for a highly competitive term like 'seo' or a top 10 ranking for a lesser competitive word like 'seo tutorial'. Given the fact that most people who use search engines almost never go see after the top 20 results, consequently a poor ranking for a highly competitive search term (like 'seo') will drive much less traffic to your web site compared to a top 10 ranking on a less competitive term (like 'seo tutorial').