Search Engine Optimisation is the act of formatting a web page(site) so that a search engine sees the page as you intend.
Search engine optimisation asks you - "What can we do to optimise this web site so that search engine spiders will find it easily, crawl it often, have a high page rank, and lots of visitors?"
Light Design includes Search engine optimisation in the original foundations or planning stages(the best time to think about search engine optimisation)of your site, and through the copy writing process.
Your web site is open 24 hours 7 days a week 365 1/4 days of the year (provided you have a good web host) and your site needs to be advertising your words/work every second of that time.
Search engine optimisation involves high quality web pages with good meta descriptions and keyword-rich content, it should be incorporated naturally into a website from the ground up.
Search engine submission (making sure search engines know your site exists) and search engine marketing come after your website is set up.