From the English Wikipedia about Telegram,
https://en.wikipedia.org/wiki/Telegram_(software)

Telegram is a cloud-based instant messaging service. Telegram clients exist for both mobile (Android, iOS, Windows Phone, Ubuntu Touch) and desktop systems (Windows, OS X, Linux). Users can send messages and exchange photos, videos, stickers and files of any type. Telegram also provides optional end-to-end encrypted messaging with self-destruct timers, but Telegram's implementation of these features has been contested by security researchers and cryptography experts.

Telegram is supported by the Russian-born entrepreneur Pavel Durov, who is now a citizen of Saint Kitts and Nevis, travelling the world in self-imposed exile. Its client-side code is open-source software, whereas its server-side code is closed-sourced and proprietary. The service also provides APIs to independent developers.