How Do Search Engines Determine Rankings?

While the algorithms the search engines use are proprietary information and nobody is completely positive how much weight the various factors are given or even exactly what factors are used, there is a general consensus with regard to which factors are used and their relative importance.

These are tried and true factors which help with SEO. These factors are the basics that have not changed over the years and seem to produce the greatest benefits.

1. Keyword density. It is important that the keywords you wished to be ranked for actually appear on your page. However, if they are used too often, the search engines will consider it spam. A keyword density between 8-12% (including words used in your tags) would be a reasonable use.
2. Keyword position on page and paragraph. Search engines try to simulate human behavior. Because of that, they consider the content (words) at the beginning to be more important and a better clue to what the page is about than the content at the end of the page. (Think about how you read. If what you are looking for isn’t at the beginning, you will move on.) Similarly, the first sentence of a paragraph is important. The topic sentence of a paragraph describes what the paragraph is about so if the keywords are in that sentence, it indicates to the search engines the paragraph is about those words.
3. The title tag. The title tag is the most important of the tags (title, description, and keyword). It is the first thing the search engine spiders see and it is the first clue they get  as to what the page is about. It should be short and describe what the page is about. In some cases, the title tag alone can get you top rankings for a desired keyword/phrase.
4. Words in bold, italics, and H1 tags. Having a word in bold, italics, or in an H1 Tag draws attention to the word. Since these words are stressed, they are usually important so the search engines take this into consideration. Similarly, some extra weight is given to list/bullet points. However, if these “word enhancements” are over used, it will be considered spam by the search engines.
5. Number of incoming links. All search engines (though Google seems to give this more weight than others) consider an incoming link a “vote” for that page. Search engines figure the more “votes” a page gets the more important the page, so the higher it should be ranked. However, not all “votes” are the same. Votes from contextually relevant sites will be worth more than votes from unrelated sites. Additionally, the anchor text of the link is taken into consideration since it gives the spiders the first clue what the page they are going to is about. “Click here” is not as valuable as learn more about link building.

These factors are given some weight, but are not as important.

1. The description Meta tag. Some search engines will use the description tag as the description of the page on the search result page. The description tag is important and should be utilized; however, it is not as important as the factors listed above.
2. The age of the domain name. The older the domain name, the more it can be trusted since it has been around longer. However, if the focus of the site changes, it will lose this gained trust.
3. The number of visitors a page receives and how long the visitor stays at the site. The reasoning is if your site contains valuable information more people will go to the site and will stay there to read what you have to say.