Some of them are:
The Meta Keywords, Description and Robots tag. — Google used description copy to market your site in its results, treat this as optimized sales copy. Make sure your sitemap.xml page is CONSTANTLY called up to the search engines
The Title Tag,The Body Text ,The first sentence in the body text and the first words used per sentence are the most important ones for advanced on-page SEO necessities,the URL should contain useful links.
The H1 and H2 Texts are highly important .
Same Site Link Texts .
Same-Site Link URLs .
IMG Alt Tags — Your images should be described via ALT Tags with the keywords you are optimizing for - If your image is hosted on your web-host, the file name of the images should also be SEO’d — If you have an image about polar bears have it be polar-bear.jpg in your images folder.
Keyword Density .
Your HTML comments should have your SEO terms in them .