In-app notifications are UI patterns used to communicate key information with users while they interact with a product. They can enhance user experience, allow for testing and optimizing flows, push forward freemium conversions, and retain users, improving LTV. There are different types of in-app notifications such as tooltips, modals, banners, slideouts, product tours, product updates, and microsurveys. To effectively use in-app notifications, it's important to follow best practices like greeting and educating new users, segmenting different audiences, being specific but not over-communicating, always providing value, encouraging users with a CTA, gathering user feedback whenever possible, keeping messages relevant to the user's behavior, and making it easy for users to exit.