Dropbox and Copy are two similar services that allow users to store files in the cloud and sync them to a folder that is on your desktop. Each service has an online interface where you can upload and manage files. You also have the option to download an application which places a folder on your hard drive that is tied to your cloud storage. Any file that you put in that folder is accessible locally on your device but is also synced to the cloud. You can also access your files using an iPad/iPhone app.
I use both accounts and store documents and images for a blog on one account and personal files on the other account. Great way to keep things organized.