Number of links aka "votes" to your website from other relevant and good websites, the age of your website, if your site has unique content, the sites you link to and so on...
As written above. - Age of domain/site. - Amount of links - Relevance of inlinks. - Links from your site to other relevant sites. - Content relevance The content means: Title, h1 tag, strong tag, em tag, the links. Don't repeat your keywords as hell, just let it slide in a few times with the text. The inlinks will do the most job.