SEO refers to Search Engine Optimization in which a particular search engine uses a particular format or a procedure to rank your sites. Say for instance in the medical field there maybe websites of your competitors as well.
So the search engine uses some format to rank you among all your competitors and therefore it is always advised to have a medical website which is search engine friendly. SERP are the research pages formed after a search has been performed. It refers to Search Engine Research Page. SEO could be used for a variety of online businesses, from rave retailers to book resellers.