Microsoft decided a while back to stop jealously guarding its popular productivity software and create proper apps for Android and iOS. The Office apps first came to Android for tablets only, but a phone preview started a few weeks ago. Now phone support is live for everyone, so go grab your free Word, PowerPoint, and Excel apps.
These are (thankfully) not new listings in the Play Store—the tablet and phone versions of each app are bundled together. If you have yet to use Office on an Android tablet, it should be more or less the same experience. You get most of the basic tools from the desktop in a touch-optimized package. You can use it for free, but without an Office subscription you won't have things like track changes and custom headers.
Microsoft will also be pushing OEMs to pre-load Office apps on phones, similar to what Samsung did with the Galaxy S6. You should be seeing more phones ship with Word, Excel, and PowerPoint later this year.