Its a procedure which helps search engines(Google,Bing,Yahoo) find and rank your web page or website higher than the a million of other sites.
Actually SEO focuses on rankings in the organic (non-paid) search result.
Types of SEO-
White Hat SEO-
Black Hat SEO-The Word Black Hat SEO is usually refers to use strategies that violate Search Engine guidelines (these are the guidelines Search Engines provide to let us know what is the right way to improve your website or web page ranking) to increase website Rankings and Traffic. These tactics may work in the short period,
keyword stuffing,buying links,paid social bookmarking etc.
Basically when we changes or update anything in our website ,its comes under on-page SEO