Cloud drives are the technology used for backup to store files and programs remotely to another computer generally through the web. Such services include Zumodrive (often used by HP), Amazon Cloud Drive, Windows Live Skydrive, Apple ICloud (for IOS.5 users), and so many others.
Although they vary in amount of space offered (the above offer free accounts), the services vary a bit between them.
Collaboration and sharing– Some offer a method to share what you have in your cloud drive such as documents or pictures, while others like the ICloud do not. You can often email the files, but a ‘cloud’ enables you to allow guests to see your files and even edit them, if you set permissions.
Sync to desktop– The above mentioned do include a method to share the files with a program to sync between the cloud drive and your computer’s hard drive. Some offer Iphone/Ipad apps to also sync your files.
Types of files– Most of these cloud drives allow for music or audio files, movies files, documents of various types, pictures of various types, and only limit the size of each file.
For further reading, check out Cloud Drive Comparisons. This articles includes details on cloud drive sizes and file formats.