Duolingo is a free, gamified language-learning app and website that offers courses in over 40 languages, including Spanish, French, German, and even constructed languages like Esperanto and Klingon. Launched in 2011, it uses bite-sized lessons with interactive exercises, such as translation, listening, and speaking tasks, to teach vocabulary and grammar. The app employs a reward system with virtual currency (lingots or gems) and streaks to keep users motivated, while its adaptive algorithm tailors lessons to individual progress.