SEO, or Search Engine Optimization, involves improving website visibility on search engines like Google and Bing to drive organic traffic. It differs from SEM (Search Engine Marketing) and PPC (Pay-Per-Click) in that SEO focuses on unpaid traffic while SEM includes both paid and unpaid strategies. Successful SEO relies on understanding search engines, conducting thorough research, and implementing effective strategies across technical, on-site, and off-site optimizations.