SEO is a process to improve the ranking(visibility) of website in search engines result pages(SERP). It provides with the techniques by which the websites can rank well on the Search Engines. The web site is displayed higher in a search engine list (like Google), then it will recieve more visitors.
It includes different methods to optimize(make accordingly) the website for search engine and for the users by adding keywords, meta tags, relevant content etc. which are required according to the guidelines given by search engine (like google).SEO considers how search engines work, what people search for, and which search terms (words) are typed.
There are types of SEO techniques.
On-Page SEO.
Off-Page SEO.