Duolingo is a popular language-learning platform and mobile app that offers free and paid language courses. It was created by Luis von Ahn and Severin Hacker and was launched in 2011. Duolingo provides a gamified approach to learning languages, making it both fun and effective for users of all ages.

Key features of Duolingo include:

1. Language Courses: Duolingo offers a wide range of language courses, including major languages like English, Spanish, French, German, Chinese, Japanese, and many others. Users can select the language they want to learn and proceed through various lessons.
2. Gamified Learning: Duolingo uses gamification elements to keep learners engaged and motivated. Users earn points and rewards for completing lessons, achieving streaks, and reaching certain milestones in their language learning journey.
3. Bite-sized Lessons: The platform uses short, interactive lessons that cover different language skills, such as vocabulary, grammar, listening, speaking, and reading. This approach allows users to learn at their own pace and fit in language practice into their daily routine easily.
4. Translation Tasks: Duolingo incorporates translation exercises, where users can practice their language skills by translating sentences from the language they are learning into their native language and vice versa.
5. Crowdsourcing: Duolingo operates on a freemium model, offering its courses for free to users while generating revenue by allowing users to translate real-world texts as part of their practice. This makes Duolingo a form of crowdsourcing for translation tasks.
6. Skill Levels: Users progress through different skill levels as they complete lessons and exercises. The difficulty gradually increases as learners become more proficient in the language.
7. Social Features: Duolingo allows users to connect with friends, compete on leaderboards, and share their progress on social media, fostering a sense of community and friendly competition.
8. Mobile App: Duolingo is available as a mobile app, making it convenient for users to learn on the go and practice their language skills anytime and anywhere.

Overall, Duolingo has gained widespread popularity due to its accessibility, interactive approach, and effectiveness in helping learners develop basic language skills. However, it’s essential to remember that while it can be a valuable tool, achieving fluency in a language typically requires a more comprehensive and immersive language-learning experience. Duolingo can serve as a useful supplement to other language-learning methods, such as immersion courses, language exchange programs, or conversational practice with native speakers.