Last.fm is a music recommendation and streaming service that uses a music recommender system called 'Audioscrobbler' to track users' listening habits and suggest new music based on those patterns. It allows users to create a personal music profile and offers features like creating playlists, joining groups, and discovering new music through recommendations and compatibility with other users. A subscription to Last.fm, known as Last.fm Pro, typically provides features like an ad-free experience, access to advanced music statistics, and the ability to view recent visitors to your profile.