Half and half is indeed half heavy cream and half whole milk.
It has a fat content between about 12 and 20% (I believe heavy cream, or heavy whipping cream, weighs in at over 30%).
This fat content means it is ideal, like Paulette said upthread, to make very rich foods/drinks "less rich" or not-so-rich "more rich."
The problem with using it as a whipped cream is that it does not contain enough fat to make a heavy, stable, delicious foam. It *will* whip, but do a side-by-side comparison and note that the whip made with half and half will not be as glossy, firm, or stable as the whip made with heavy--and it will begin to "weep" much more quickly. Of course, it will do in a pinch
Half and half is a common ingredient in cocktails, such as White Russians, but as you said upthread, it is used most often for those who like to cut their coffee. The problem with using it in sauces is that it can't take a boil like heavy cream can, there is not enough fat floating around in it to keep it from curdling. Even when using heavy cream in a sauce, I always like to temper it a little bit before adding it to the hot pan.
But I'm a paranoid bastard.