It is also 'Pay Per Click' (PPC) which is text advertising delivered in response to 'trigger' words typed into the search box.
SEO is Search Engine Optimisation. It is the process of building pages in a manner which enables the pages to rank highly in 'algorithmically' determined search results, which are generally displayed on the left hand side of the page. The 'algorithmic' results are independently determined by the search engine's own mathematical algorithm, and are based on relevance to the search phrase.