2 Types of SEO

Essentially there are 2 types of Search Engine Optimisation:

1. on-page / on site SEO

2. off-site SEO

On page concentrates on the text within the webpage/website, the various tags and elements etc within the webpage that we can use to let the search engines know what the page is about.  On site SEO covers things like site architecture, internal linking structure, internal anchor text (what wordage you use in your links), page titles, headings, descriptions and keywords etc.

Off-site SEO is about getting relevant links to your web pages from respected resources to reinforce your on-page / on-site factors.

In essence off-site SEO is about getting other websites to recommend your website as a trusted resource on a particular subject by linking to it.
It is the word-of-mouth that increases the popularity of your offering.  In other words – you create relevant meaningful content and then other people tell the search engines how good it is.

The on page / on site factors are easily controlled by the website owner and therefore generally not weighted as highly as ranking factors as the external off-site factors (although the weightings do shift all of the time).

So, in order to rank as well as possible a good SEO effort will optimise the on-page and on-site factors and then go about building links into the site to create some link-popularity.