Technically speaking, Search Engine Optimisation refers only to carrying out various activities to help a website accessible to a search engine and giving prominence to selected keywords. Typically this just means ensuring that each page has a relevant and descriptive page title and meta description, and that the site is valid HTML and has no broken links.
However, SEO has become a catchall term used to describe a huge variety of activities - from copywriting, to HTML validation, to link building and more.
In fact, because the term is so poorly defined, many website marketing companies are trying to use different definitions - concentrating on specialities that are sometimes said to be part of SEO - such as linkbuilding or copywriting.