SEM = search engine marketing. SEM encompasses both SEO and paid search.
SERP = search engine results page.
PPC = pay-per-click. Otherwise called paid search.
SEO = search engine optimization. SEO focuses on the organic, or unpaid, listings that appear naturally on a SERP.
ROFL = rolling on floor laughing. What you should do when the Yellow Pages promises you "top organic placement in all major search engines."