The Importance of Rest for Muscles
If you see a body builder in the gym every single day, take a look at what muscles he or she is working. You will notice that he or she does not work the same muscle groups every day. They rotate days so that each group has an appropriate amount of rest time before being worked on again.
You don't have to be a body builder to understand the necessity of muscle rest, though. Even if you are not necessarily trying to "bulk up" (you may be trying to do just the opposite!), imagine the damage you could do to your muscles if you haven't allowed the tears from your last workout to properly heal.
When your muscles are sore or feel exhausted, even though the rest of your body feels fine, it means that they have not fully recovered yet. Stretching assists the muscle healing process by improving blood flow to the muscles, which is why it is so important to stretch not only before each workout, but after as well.
So how much is enough -- rest that is? The period of rest will vary greatly from person to person, and from muscle group to muscle group. In general, abdominal muscles are the quickest muscles to recover, and can be worked out every day (apologies to all those who were praying for a break from abdominal exercises!).