I am going to start with SEO, as it is a part of SEM. SEO is the act of optimising the HTML and other content of your website for relevant, targeted key phrases in order to attain higher natural listings than competing websites. SEO provides a cheaper long term solution for increased qualified traffic and generates customer inquiries that ultimately convert to sales. Now SEM is broader than SEO. It includes SEO and other areas to improve a sites visibility in search engine results pages, like paid listings and paid inclusions. You can think of SEM as more expensive and quite possibly more targeted, while SEO is free (not counting your time of course) and its purpose is to obtain better free search listings. Personally, I would start with SEO and then move onto SEM.