What do search engines use to rank a web site?

Link Popularity
How many sites have links to your site? Your site gets points from a search engine for the popularity of the site that has a link to yours and the number of sites that have links to your site. Links from seldom-visited sites to your site help a little, but receive a lower score from a search engine optimization point of view. To get the best ranking include many links from often-visited sites. Another element considered in ranking links is relevance: you get more points for sites whose topic is relevant to yours. In other words, if your "group exercise classes" site had a link pointing to it from a major physical fitness site, that would be more relevant and therefore worth more than a link from a major unrelated site.

Keywords in Meta Tags
In the past, one of the most important things a web designer could do to help the ranking of a web site was to create a line of code that wouldn’t show but would contain a list of keywords. These were known as META-Tags keywords. Now most search engines don’t look at META-tag keywords at all. They are, however, still worth using because several search engines still use them.

What makes it difficult for search engines to rank a web site?

The following three examples won’t get your site banned, but may hinder your sites placement on search engine indexes.

Non-HTML links
Some search engines can’t follow links that are not html hypertext links, so pages within a non-html site can’t be crawled. The problem can be solved by using a site map with html hypertext links and an html hypertext link on each page to that site map.

Frames
Pages that use frames can be difficult for search engines to crawl. Although many search engines have become more sophisticated and can now crawl well-designed sites utilizing frames, they must be coded carefully, keeping search engines requirements in mind.

Dynamic Content and Multi-media
Many search engines cannot index dynamic content (URLs with "?" in them) and multi-media files (most commonly Flash). A common work around is to create search engine-friendly simple html pages to improve the ability of these sites to be indexed.