From Open Whisper Systems:
Say Anything – Send high-quality group, text, picture, and video messages. Be Yourself – Signal uses your existing phone number and address book. There are no separate logins, usernames, or PINs to manage or lose. Stay Private – We can’t read your messages, and no one else can either. No exceptions. Everything in Signal is always end-to-end encrypted and painstakingly engineered in order to keep your communication safe. Pay Nothing – Signal is supported by a team of dedicated developers, community donations, and grants. There are no advertisements, and it doesn’t cost anything to use.