Every site is given a pagerank (pr) number from zero to ten with yahoo and google being a ten and new domains with no links being zero. The higher the number the easier it is to rank for any given keyword. Pagerank is calculated from a complex formula that few other than google engineers can understand. The basic rule of thumb to increase pagerank is to get links from other websites with a high pr and limit the number of links going out to other sites.
2. Incoming links
The quantity and even more importantly the quality of links is an important factor. Google looks for sites that appear to have a natural link popularity. That means having a wide variety of sites linking not just one type such as social bookmarking or blogs. Also the link text is important. If all incoming links are the same keyword it is a red flag and will likely hurt more than help rankings. Relevancy of the links is also important. A link is relevant if it is coming from another site of the same topic. Finally there is the the rate of link growth. What Google likes to see is a combination of slow link growth from a variety of different relevant sites and varying link text.
3. Page titles
This is the most imporant part of the page that the algorith looks at. It is important to choose keyword rich titles for each page.
4. Keyword density
The number of times a keyword shows up on a given page helps tell the search engine what it's about. If keywords are used too many times then it is a sign of spam to google and will actually hurt the sites ranking. A good idea is to keep keywords density lower and instead use related keywords. That is known as latent semantic indexing (lsi) and google favors sites that use it.
5. Age of domain
The actual age of a domain can be a friend or enemy for search engine optimization. Newer domains are very difficult to get ranked while domains older than two years are much easier. This is because older domains are trusted more.
While meta tags used to also be a the five factors above are the main reasons why certain sites show up often on the first page of the results on google.
