Here's how I would sum it up...
Search Engines Scrutinize You and Check Your References
Search engines look at both "on page" and "off page" factors. This means they look at each of your pages (they look "on page") for how they are designed, programmed and authored. They look at how the pages of your site connect together. And mostly they are looking for which keywords are located where on each page.
They also look "off page" at other sites to see both how many sites link to you, and how those sites relate to your topic and keywords.
Search Engines Optimization begins with the Right Keywords
Everything about your online marketing, from the topics you cover, to the names of your files, to the text you write, should all be based upon your list of target keywords. At first you should focus on the most actively searched keywords that relate to what you do in your market. Over time, a good SEO firm will help you refine the list to the terms that are most likely to attract a new patient.
SEO in a Nutshell
So, in a nutshell, SEO is about first choosing the right keywords, using them judiciously in well written text, on a well built site, and then working to secure lots of high quality inbound links from related sites.