I found myself wondering about this, so I did a little research around the web and found some info on superwash wool from Lion Brand:
"Superwash wool is a wool yarn that is machine washable and, therefore, will not felt. Each hair of wool is made up of scales. Felting occurs when these scales bind together. The superwash process prevents the scales from binding in one of two ways. Some superwash wools are given an acid bath that removes its scales. Alternatively, the yarn can be coated with a polymer or resin; this is essentially a protective coating for the yarn to prevent felting. A yarn can be treated with either or both methods to become superwash.
It's important to remember that excessive heat (such as with a hot setting on a washing machine or dryer) can damage a superwash coating, which may lead to felting. That is why we recommend cold washing and flat drying with our superwash wools. Also, keep in mind that superwash wools tend to stretch a little more than normal. This is because the scales of the yarn cannot bind together. It's especially important to do a proper gauge swatch with a superwash wool to see how your yarn will stretch.
A final thing worth noting is that not all washable yarns are superwash. This is because superwash is a patented process. Washable wools that are not superwash may have very different washing and drying instructions, so it's important to always follow your yarn label's care instructions."
Interesting. Thank you, Lion Brand for providing that information! I've definitely noticed that my finished superwash wool pieces stretch a lot more than plain wool pieces. This could potentially be seen as an advantage if you are slightly unsure of the size of your recipient's head or feet when making a gift. In any case, it's always good to know the facts about the materials you are using.