OneDrive is Microsoft's cloud storage service, recently renamed from SkyDrive. It has a generous free level, offering the same 15GB free as Google Drive, plus 3GB extra if you sync your photos from your mobile device with it, and another 500MB per referral. If you use Microsoft Office Online or OneNote, your OneDrive is the default place that your files are saved.
OneDrive truly is "one" because it runs on everything. If you are a Windows 8.1 user, it's already built in. For other platforms, both desktop and mobile, the client is an App that you install. It is even available for the Kindle Fire in the Amazon App store, for Chromebooks in the Chrome Web Store, and for the XBox, and of course, for Windows tablets and phones. You'll use your Microsoft account as your credentials; if you already use Outlook for your email, that's your Microsoft account, otherwise, signing up is free and quick.