In order to run the most effective Search Engine Optimization campaign possible, you need to be concerned with three main areas.
It is possible to rank well by focusing on only one or two of the following, but in the long run your results will be better and longer-lasting if all three categories are improved at the same time.
When you perform a search on the Google website (or Bing, Yahoo Search or any of the myriad of other online search engines), you are not literally "searching the web" at the moment.
You are actually performing a request for Google's servers to search through its index, which is a database holding a subset of all of the sites and other documents accessible on the World Wide Web.
The three areas of an effective SEO campaign will make sure that...

1. Your site in included in Google's index
2. Your content page is considered relevant for the search being performed
3. Your site is considered trusted and authoritative by a sufficient number of other websites

The Three Areas of SEO Assuming that you know your market and which searches you want to rank for, the three areas to work on to get the most improvement can be summarized by these three R's: Technical SEO - This area is concerned with the technical aspects of your website; all of areas that will either help or hinder your site being crawled and indexed.
Most of these aspects are like a pass/fail class; if they are not configured properly, they are probably preventing your site from reaching its full potential in the search engines' results.
Relevance - This area of SEO is affected by the indexed content of your site and how that content is presented and organized.
In order to rank for a certain search phrase, it is important to use the exact phrase in the important areas of the page you want to rank best, and to use them in such a way that it is obvious that the search phrase is the main topic of the page.
For general phrases that are related to multiple pages of your website, organizing the content properly can help Google realize how much valuable content the site has about the topic.
Your site is probably about one main topic and multiple subtopics.
The best and easiest tactic to create relevance is to take the time to create the best content you can around those topics.
Links from Other Sites - Google builds trust in a website by analyzing the links pointing to the site from other web pages.
When one site links to another, it is akin to vouching for the content contained on that website.
Each link is a pointer showing where the visitor can find additional information about a topic.
The best way to develop links of the highest quality is to develop relationships.
Rather than artificially creating links, building relationships provides a valid reason for others to link to you.

• Provide great service to your customers so they blog about you, recommend you on Facebook and help to spread your reputation with word-of-mouth advertising.
  
• Get closer to your suppliers - and especially manufacturers - so they provide a "where to buy" link to your site
• Provide great content, such as reviews or buyers guides, to help prospects make a wise buying decision (and encourage others to link to your website's content at the same time).
  
• Share your expertise; offer to guest blog on others' websites.
  
• Link to other websites that sell products or services complementary to your own in return for a recommendation of your own offerings.
  

The first area mentioned above is not optional.
If you neglect a technical aspect that is preventing your site from being fully crawled and indexed, your pages may not rank well no matter how relevant the content is or how many links you have pointing to your site.
But by making the effort to develop all three of the areas mentioned above, your chances of climbing in the search engine rankings are much greater than if you focus on only content or only link building.