iDisk was Apple’s online disk space that you get automatically if you sign up for MobileMe. It has been discontinued. The easiest replacement is probably dropbox.
Apple has chosen not to migrate iDisk across to iCloud at this stage, but a good replacement is dropbox.
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You sign up online, then it makes a folder on your hard disk that is always ‘synced’ with the online version. You can then choose to share individual files with others by making them public if you choose.