Telegram is a non-profit cloud-based instant messaging service.
Users can send messages and exchange photos, videos, stickers, audio and files of any type.
Its client-side code is open-source software but the source code for recent versions is not always immediately published, whereas its server-side code is closed-source and proprietary.
The service also provides APIs to independent developers.

This is the official binary built by Telegram devs, but the package is unofficial.