Search Engine Optimization (SEO) is the process of choosing targeted keyword or phrases related to a site, and ensuring that the site ranks well when those keyword phrases are part of a Web search.
Search engine optimization is often confused with search engine spamming. There are some clear examples of both, but there is a gray area in between. SEO helps users to find your content online, while spamming is when your website gets listed in search results for keywords without any value.
Optimization involves ensuring a website is optimized so that it is visible to search engines. A good example of an optimization is ensuring a website is not part of the invisible web (the portion of the web that is not visible to search engines). The more advanced optimization is when it involves extensive research into the website design, the structure, and the off-site elements.
Before your pages can be optimized for search engines, you must first research the keywords that are relevant to your niche. You must assess their popularity, competition, and determine which keywords can be supported with good content.