Telegram and Signal are widely popular privacy-focused messaging apps. However, the two apps have some big differences: While all messages sent via Signal are always end-to-end encrypted by default, Telegram messages aren’t. End-to-end encryption is an optional feature in Telegram.
End-to-end encryption means that only the sender and the receiver of a message can see its contents. Not even the company running the server in the middle of the conversations can see the contents of the communications.
With Signal, all conversations are always end-to-end encrypted: The Signal Foundation can’t see the contents of the messages.
With Telegram, the company in charge of Telegram is technically capable of seeing the contents of the messages on its servers.