This is a pretty good article that explains the basics of Android version 4.0, aka Ice Cream Sandwich or ICS. If you are already experienced with Android it will probably not be all that helpful.
More useful if you are experienced with Android, this chart helps you find where to do things in ICS. I know for me it was something of an adjustment when I moved from Gingerbread (2.3) to ICS as a number of everyday things were changed, e.g. adding widgets. So this was useful to me.