Of course, WhatsApp has had a local backup feature for years, but without any way to automatically upload that backup to the cloud, its usefulness was rather limited. If your phone ever got lost or stolen and you didn't have a backup stored elsewhere, your entire chat history would be gone forever. There were ways around this limitation (e.g. using FolderSync to upload backups to Google Drive), but this added a layer of unnecessary complication that very few users bothered going through.
Thankfully, the new process is very straightforward and quick to set up. You can back up your chat logs, photos, voice messages, and now even videos to your Google Drive account. Additionally, since it doesn't rely on Marshmallow's new backup capabilities but instead on a Google Drive API, you should also be able to get it working even if you're part of the 0.2% of users still running Froyo.
Backing up to Google Drive
The setup process is essentially identical to the one we described a few months ago. After updating the app to a compatible version, you'll be greeted with a prompt to configure settings such as backup frequency, Google account, and whether or not to upload videos.
Restoring a Backup
Restoring a backup to a new phone should also be a relatively painless process. After confirming your phone number, WhatsApp will check your local storage (in the WhatsApp folder) for any backups. If it can't find any, it will check your Google Drive instead and restore your data from there. Thankfully, the message database is downloaded and restored before the rest of your media, so you won't need to wait for half a gigabyte to download before you can begin WhatsApp-ing with your friends again.
Of course, something always has to go wrong. After a couple of attempts at processing my backup, WhatsApp was unable to restore my message history. It still managed to download all my media, but in my case, the message database appeared to be corrupt and, therefore, impossible to restore. Fortunately, I had another database backed up locally, and I was able to restore everything from there after one more try.