Telegram is an instant messaging app that, like similar apps such as Viber, WhatsApp, and LINE, gives you a simple, accessible way to communicate with all of your contacts. The main difference between them is that Telegram also offers you increased security and privacy.
Telegram's security options allow you to create private chats with other users, which means that your conversations with them are completely encrypted and aren't saved or stored on Telegram's servers. And you can activate a "self-destruct" feature so that your messages are erased every X amount of time.
Aside from those privacy features, Telegram offers basically the same service as any other instant messaging system: group chats, file sharing, personalized notifications, etc.
Telegram is an instant messaging tool that shares many similarities -including its appearance- with WhatsApp. Still, the additional privacy that it provides could be very attractive to many people. Don't want conversations with certain contacts to be saved? Set them to self-destruct after ten seconds, and you're all set!