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. (Note: there’s now a better way using Apple Message that I describe here.)
Apple has chosen not to migrate iDisk across to iCloud at this stage, but a good replacement is dropbox.
Check it our here:
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.
Once you’ve copied a file into your dropbox ‘public’ folder, you can right click on the file and select ‘Copy Public Link’.
This will give you a link (URL) that you can paste into an email and send to someone. When they click on the URL, they will be able to access your file.