What is SEO?

Basic

Search Engine Optimisation (SEO) the process of improving a websites ranking in search engines (un-paid or natural results). By nature, the higher a website is in the search rankings for key terms the higher the number of views that website will get in not only the search results but also the sites home page.
SEO can target many things, The natural text results, image search, local search, video search and other industry specific search engines.

How it Works / Techniques

SEO is done in many different ways, some are changes that need to be made on the website, and some are changes that need to be made off site. Some of the most basic ways to perform SEO are changing Keywords, Texts, Meta Descriptions, Meta Keywords, Headings, Image ALT Tags and others.
Off Site techniques include Link Trading, XML Site-map Submission, Directory Submission, Search Engine Submission, Making Relevant Forum Posts, Writing Articles, RSS feed to blogging sites and more.