A new Google app that allows you to text your friends? Wait, is this replacing Hangouts? Wait, why? Yes, no, and because, to answer those queries, respectively. Google Messenger is now out on the Play Store, having been officially confirmed to exist by Google last month.
This new app is the stock SMS/MMS handler in Android 5.0, but also can be downloaded to other devices with SMS/MMS capabilities, provided they run Android 4.1 or higher.