Telegram is like SMS, but more powerful. You can send messages, photos, videos and documents to people who are in your phone contacts (and have Telegram). You can also create groups for up to 200 people.
Unlike WhatsApp, Telegram is cloud-based and heavily encrypted. As a result, you can access your messages from several devices (including desktops!) and share an unlimited number of photos, videos and documents (doc, zip, mp3, etc). 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.