Content: The more unique, well-versed content a site has, the better it will rank.  Content primarily consists of the text on your site, but also includes images, videos and anything else the users come to the site for.

Links: Mainly concerning inbound links, but also includes outbound links and internal links.  The more, quality, inbound links a site has, the better it will rank.

Popularity: The more people that know about your site the better it will rank and the more traffic you will receive.  Popularity is often measured by the number of inbound links to a site.

Reputation: Popularity alone is worthless without a good reputation.  If a site is known by many to be of poor quality or of little to no use to the users it will not rank well.