The name Lernu comes from the imperative form of the Esperanto verb lerni, meaning "to learn".

The site's content includes various tools for learning Esperanto, such as exercises, games, dictionaries, grammatical overviews, an examination system and a multimedia library with books, music, voice-narrated stories and videos.

The site also provides communication services such as an instant messenger, discussion forums on various topics, and news about Esperanto events.

In July 2002, the project received monetary support from the American Esperantic Studies Foundation (ESF), and work on the website started in August of the same year.

There are multiple courses on the website, ranging from simple word learning (Bildoj kaj Demandoj) to difficult rewriting of stories (Kio okazas) Users are able to view the site's interface in their choice of 24 languages – Catalan, Chinese (both simplified and traditional characters), Danish, English, Esperanto, Finnish, French, Georgian, German, Hebrew, Hungarian, Italian, Kirundi, Kiswahili, Norwegian (Bokmål), Persian, Portuguese, Romanian, Russian, Serbian, Slovak, Slovenian, Swedish and Ukrainian; a further five languages—Bulgarian, Croatian, Czech, Indonesian and Spanish—have at least 70 percent of the interface localized; nine additional languages – Dutch, Greek, Japanese, Korean, Lithuanian, Polish, Thai, Turkish and Vietnamese – are in varying stages of completing the interface translation.