Search engines change every day, especially those that have the largest infrastructure and budget to improve their technology. Marketers who understand the need for constant testing to improve their performance will fit in very nicely with the main concept of search engine optimization: Deliver the right content to the right people when they are searching for it. Search engines explore the Internet constantly looking for new content to index and then use an algorithm to rank that content within “organic,” or “free,” search results, unlike directories, which simply gather and list Web pages within categories.