A iDisk is a disk space on Apple servers reserved for apple user. If you have MobileMe By default this disk space of 20 GB (20,480 MB ) is shared between your iDisk and your mailbox me.com according to your setting configuration. For example, 18gb for iDisk and 2gB is for your email box. You can change this by configuring your account on the web page of MobileMe .
A iDisk works like an extra partition. You just have to mount your iDisk on your desktop that will become new partition.You can easily use this space by your way, you can download required application simply by drag and drop software onto your desktop.
A iDisk can be very useful, specially with the folder Public in your space. The data placed in the Public can be viewed and downloaded by people whom you have given your Login so shearing is easy. You increase 20gb space with your annual subscription to MobileMe,you can customize the size depending on your needs.
On MAC OS X, it is possible to use iDisk as local disk offline as it is possible to make a copy of iDisk on your desktop and when you start Internet mac automatically synchronize the changes you can do that manually also .
it is possible to use a feature for sharing files on the Internet, directly from your account MobileMe via me.com site