Recommendation engines help companies recommend the right products or services to customers based on their past behavior. They work by discovering patterns in customer data using machine learning and recommending items that correlate to customer interests. There are three main types: content-based filtering which recommends similar items; collaborative filtering which recommends items liked by similar users; and hybrid models which combine both approaches. Amazon and Netflix are examples of successful companies that use recommendation engines. Amazon's engine contributes significantly to its sales growth, with high conversion rates from email recommendations. Netflix's proprietary system analyzes streaming data to provide personalized movie and show recommendations to help users find new content of interest.