SEO involves optimizing websites to rank higher in search engine results pages. There are three main types of SEO: on-page optimization of elements like titles and metadata, off-page activities like link building and social media marketing, and technical SEO to improve site speed and functionality. Effective SEO strategies can be divided into white hat best practices like high-quality content, black hat manipulation like hidden text, and grey hat tactics in a legal grey area. Keyword research is important to identify popular search terms to target with optimized content.