Wikipedia
iDisk was a file hosting service offered by Apple to all MobileMe members that enabled them to store their digital photos, films and personal files on-line so they could be accessed remotely. With a standard subscription, MobileMe users received a 20 GB iDisk.
iDisk integrated with Mac OS X, appearing as a network drive. Mac OS X v10.3 through v10.6 could cache updates to an iDisk volume while off-line and synchronize updates later. Any WebDAV client could also access an iDisk volume.
From 2000-2002, iDisk was free for all Mac users through iTools, a group of services that evolved into .Mac, and then after a name change in 2008 became MobileMe.
Apple discontinued MobileMe on 30 June 2012 and iDisk was no longer available. While Apple did not transition the iDisk service to iCloud, they have recently announced iCloud Drive as their next cloud storage service incorporated into OS X Yosemite and iOS 8.