SEO is a method to optimize a website so that it will be easier to find on major search engines like Google, Yahoo or Bing.
Types of SEO
White Hat SEO
"White hat SEO utilizes techniques and methods to improve the search engine rankings of a website which don't run afoul of search engine (mainly Google) guidelines."
Black Hat SEO
"Black Hat SEO exploits weaknesses in the search engine algorithms to obtain high rankings for a website. Such techniques and methods are in direct conflict with search engine guidelines."

List of seo techniques
On-Page Factors
1. Page Titles
2. Meta Descriptions
3. Meta Tags
4. URL Structure
5. Body Tags (H1, H2, H3, H4, etc.)
6. Keyword Density
7. Image SEO
8. Internal Linking

Off-Page SEO Strategies
1. Community Creation in Social Networking Sites
2. Blogging
3. Forum Postings
4. Search Engine Submission
5. Directory Submission
6. Social Bookmarking
6. Link Exchange
7. Link Baiting
8. Cross-Linking
9. Photo Sharing
10. Video Promotions

Here is seo guide