The encryption technology was apparently turned on by default in the last update for Android devices. It was delivered all thanks to WhatsApp’s partnership with Open Whisper Systems and was implemented using their open source code.
It was a mammoth task that took about 6 months (from the time that Facebook acquired WhatsApp) to complete and deliver the tech. Open Whisper has also developed apps like Signal, Redphone, and TextSecure to name a few. WhatsApp apparently looked at TextSecure which delivers a unique future-proof encryption system for messaging that keeps messages encrypted long after they has been delivered.
WhatsApp is indeed not the only messaging service to deliver use such encryption techniques. Cryptocat, Silent Text and Telegram already offer the same features but have a much smaller user base compared to WhatsApp, which is why this is such a big deal.