Search Engine Optimization, or SEO, is the process of planning, designing and constructing a website based on the known variables the major search engines use in ranking the relevancy of your site against a keyword term or phrase.
Search Engine Optimization can bring more visitors to your website. These visitors will be "qualified" visitors, meaning that they are coming to your site based on an interest in the products or services you sell. We know this because they will have found your site based on entering in a search for the products or services you sell.
The first step in optimizing any site is to determine the most profitable keywords to optimize the site for. I do this through either my Keyword Analysis or Keyword Research services. From there, if you already have a website, I perform and on-site technical optimization of your existing pages. Finally, we start in on building new, original content and creating fresh new links from important, relevant sites pointing back to your site. This process is explained in more detail in My SEO Process section.
Search Engine Optimization can be a slow process because we have to wait for the search engine's to update their databases each time we make a change. Fortunately, we can speed that process along by letting the search engine's know each time we make important changes. After I've gotten a site ready for prime time, the first thing I do is submit a site-map indexing all of a site's relevant pages, directly to the major search engines. I've seen improvement in rankings in as little as 30 days using this method.
In a nutshell, the search engine's are interested in 1) CONTENT that's relevant to a user's search and 2) the POPULARITY of your content, based on how many other sites link to it. For a more technical, granular explanation, please see my article The Top 20 Factors to Improve Your Site's Ranking With Google.
No more than you can expect to ever get a guarantee of 100 new leads from that print ad or 20 new sales from that mailer, SEO does not come with a guarantee. Unfortunately, less ethical marketers are using guarantees to lure customers into useless service agreements. But don't take my word for it - read this article from Google on what to expect from search engine optimization.
The highest ROI I've personally seen from SEO is when I was working as VP of Marketing at Reignmaker Communications where my ROI for our SEO efforts consistently topped 400%.