The top three search engines are currently Google, Bing, and Yahoo. Once each engine finds a site then that engine will crawl through (access) each page, indexing and ranking the content. This process will continue over the life of the site.
The results are indexed according to rankings called SERPs. "SERP" is
an abbreviation for "Search Engine Results Page". The SEO (Search Engine Optimization) struggle is about getting into the top
SERPs on various search engines for specified keywords and key phrases.
Most recently, SEO has become SEM (Search Engine Marketing). It's not enough
to merely optimize the site for indexed content. The integrated approach now
encompasses three aspects: paid advertising
(AdWords) and
(Google)
analytics, site and content optimization and qualitative off-the-page factors
(in-bound linking).
Also of importance to the search engines are content depth and uniqueness. That
is, most highly ranked sites have many pages of useful information. The more
useful information provided, the better the chance of being ranked higher.
It is a common belief that Google ranks sites according to over 100 quality points. When a page is indexed the information is extracted from the site and is processed through a complex set of algorithms. The algorithms determine a site's value. Many of these ranking factors are proprietary, hidden from public knowledge. From experience we tend to know which quality factors the search engines's hold in priority and weigh more heavily.
Of most importance to the search engines is site authority. Expert consensus
is that the search engines rank a site's importance, in part, by the number of inbound
quality (authority) links.
More discussion on the above can be found on this page
How I Achieved a Ten-fold Increase in Traffic to My Sites with Quality Backlinks.