FAQ : Search Engine Indexing

Below is an excerpt from the Google wemaster help pages

How Google ranks pages

Sites' positions in our search results are determined based on a number of factors designed to provide end-users with helpful, accurate search results. These factors are explained in more detail at http://www.google.com/corporate/tech.html.

In general, webmasters can improve the rank of their sites by increasing the number of high-quality sites that link to their pages. For more information about improving your site's visibility in the Google search results, we recommend reviewing our webmaster guidelines. They outline core concepts for maintaining a Google-friendly website.