In response to a members problems in another thread where they where having problems on their windows c: drive running out of space ( due to a smallish partition). It is usefull to understand where itunes stores its data and in particular the backup data for every ios device that it control's.
The following article from apple is a useful reference How to back up your data and set up as a new device http://support.apple.com/kb/HT4137
This has become relevent to Macbook air users that have the smaller SSD drives as well as the backups can take up quite a bit of space and unlike the media files (your movies, music etc) that can readily be moved to a larger internal or external drive, the back up files cannot be easily moved.
The following article explains the backup issue quite well for Mac users. http://osxdaily.com/2011/03/05/how-t...al-hard-drive/
For windows users I have not been aple to find a suitable answer so far.
I hope you find this info useful