Web applications are interactive websites which provide some form of dynamic functionality such as Facebook. Websites on the other hand are static web pages which simply contain text/images and interact very little if not at all with users.
This is the act of making a a website accessible on the world wide web. For this to be archived, powerful computers called servers are needed. These computer have software and hardware necessary for making a website accessible on the internet.
Search Engine Optimization refers to various ways/attempts used to enhance web page rankings on search engines. Search engines use these attempts to class website and web pages as relevant and authoritative.