Telegram is a cloud based instant messaging service developed by telegram messenger LLP, a private company registered in London by Pavel durov, a Russian entrepreneur. The telegraph client application is available for Android, IOS, Windows Phone, Windows NT, Mac OS, and Linux. Users can send messages and exchange any type of photos, videos, stickers, audio and files.

The client code of telegram is open source software, but the latest version of the source code is not always released immediately, and its server-side code is open source and proprietary. The service also provides APIs to independent developers. In March 2018, telegram said it had 200 million active users a month. According to its chief executive, as of April 2017, telegram had an annual growth rate of more than 50%.

The security model of telegram has been obviously criticized by cryptography experts. They criticized the general security model, which permanently stores all contacts, messages and media and their decryption keys on their servers by default, and does not enable end-to-end encryption of messages by default. Pavel durov believes this is because it helps avoid unsafe third-party backups and allows users to access messages and files from any device. Cryptography experts have also criticized telegram for using an unproven, reliable and secure custom designed encryption protocol.

By default, messages and media in telegram are only server client encrypted. The service provides end-to-end encryption for voice calls and optional end-to-end encrypted "secret" chat between two online users, but not for groups or channels.