Dropbox is a widely-used cloud storage service that works on every system via an app or dedicated program, or via a web interface. It is easy to master, and blends seamlessly with your computer file system. It's really useful you use different devices, and want to move your files from one to another. It's also handy when you want to share files with friends and family members, or even make a file available for the public to download.
Dropbox gives you 2 GB of space for free, and will reward you with 500 GB more for each friend you introduce it to (and more for them for accepting your invitation), up to 16GB. If you need more space you can get 1TB (1000 GB) for $9.99 a month, but for most casual users, the free tier will work fine.
You can (and should) get Dropbox for all of your devices. If you have a PC or Mac, start with that to sign up and download the desktop client. Using the PC/Mac link below, and you'll get an extra 500 MB free (and so will we.)