Unlike WhatsApp a popular instant messaging service and free SMS service, Telegram is cloud-based, decentralized and heavily encrypted. As a result, you can access your messages from several devices and store an unlimited number of photos and videos in the cloud. Thanks to decentralization and encryption, Telegram is also faster and way more secure. On top of that, Telegram is free and will stay free — no ads, no subscription fees, forever.
Telegram is currently available for iPhones (iOS 5 and above) and Androids (2.2 and up). It support tablets and smartphones alike, as well as iPods (they don't have a dedicated iPad app yet, but Telegram works well on iPads too). Their also have an open API for developers, should you want to build your own applications for other platforms. In the meantime, get your best friends to switch back to iPhones and Androids.
It is Very secure. It is based on a new protocol, MTProto, built by their own specialists from scratch, with security in mind. At this moment the biggest security threat to your Telegram messages is your mother reading over your shoulder. They said they will took care of the rest.
The code available for Telegram is available on github.com. You can be able to download it from here https://github.com/DrKLO/Telegram or have a look at http://www.telegram.org their main home page.