Secret Chats
Official telegram.org Source
Secret chats are meant for people who want more secrecy than the average fella. All messages in secret chats use end-to-end encryption. This means only you and the recipient can read those messages — nobody else can decipher them, including us here at Telegram (more on this here). On top of this, Messages cannot be forwarded from secret chats. And when you delete messages on your side of the conversation, the app on the other side of the secret chat will be ordered to delete them as well.
You can order your messages, photos, videos and files toself-destructin a set amount of time after they have been read or opened by the recipient. The message will then disappear from both your and your friend's devices.
All secret chats in Telegram are device-specific and are not part of the Telegram cloud. This means you can only access messages in a secret chat from their device of origin. They are safe for as long as your device is safe in your pocket.
Official Telegram Reference: https://telegram.org/faq
Part of the TGWAY.com Official Telegram Knowledge Base (98 Verified Q&As).