Collective intelligence is defined as the intelligence that emerges from the interactions and contributions of users. It can be harnessed through allowing user interactions and contributions, aggregating what is learned about users through models, and using those models to recommend relevant content. Collective intelligence comes from both structured data like ratings and purchases, as well as unstructured data like reviews and forum posts, which are often transformed into structured data. Recommender systems are classified as collaborative filtering, content-based, or hybrid approaches. Collaborative filtering relies on user-item correlations or ratings to make recommendations, while content-based filtering analyzes item attributes.