One of Whatsapp's most bragged-about features is its end-to-end encryption. Basically, every message sent and received through the service is secure along every step of its journey – the encryption means that nobody but the sender and intended recipient can read the message. But Whatsapp's latest update introduces free backups via Google, and these backups don't use the same encryption. That means that old messages on the service might not be quite as secure as they were before.
The messages stored within Whatsapp on your mobile device itself will remain secure, but once those messages are sent to Google for backup storage, they'll lose the encryption that keeps them safe from any potential prying eyes.
In its favour, Whatsapp has been very upfront about the security limitations of Google backups. In Whatsapp's online FAQ pages about the backup service, there's an "Important" message stating "Media and messages you back up aren't protected by WhatsApp end-to-end encryption while in Google Drive."
You can manually turn on and off backups in the settings within the Whatsapp app, so you can at least choose what's most important to you. If the security of your messages is paramount, then you can set the back up frequency to never. But if encryption is less important than having a regularly-updated backup of all your weird and wonderful Whatsapp conversations, then keep the option turned on.
Google backup is only available to Android users whose phone is synced with a Google account. It seems that this function is turned on by default, so you may want to check your settings and turn it off if message encryption is most important to you. [Independent]