Google Drive is a file storage and synchronization service provided by Google, which enables user cloud storage, file sharing and collaborative editing.
Watch the video for a quick walk-through of how Google Drive works:
Dropbox allows you to share folders with other Dropbox users. Download Dropbox for easiest access, although you can also upload and download files through the Dropbox website. Android and iOS apps are available.
For more information, view the Dropbox tour.
Want more options? See Comparison of Online Backup Services in Wikipedia for a comparison chart of the various file sharing/cloud storage services available.