Duolingo is a free language-learning and crowdsourced texte translation text platform. The service is designed so that, as users progress through the lessons, they simultaneously help to translate websites and other documents.

Duolingo offers extensive written lessons and dictation, but it features less speaking practice. It has a gamified skill tree that users can progress through and a vocabulary section where learned words can be practiced.

Users gain experience points (XP) as they learn a language, such as when they complete a lesson. Skills are considered « learned » when users complete all the lessons associated with the skill.
Lessons typically comprise 14-20 questions/sentences and last between 4–7 minutes.

Duolingo_skills
A screen-shot from the English to French Duolingo