How are shingles different than a common rash?

Pain and vesicles. Shingles is characterized by vesicles, which are small fluid filled bubbles on the skin much like you'd see in a cold sore about the mouth. As well, shingles usually hurts a great deal, which most rashes do not - this pain can be debilitating at times.
It hurts! Shingles is a rash that is specifically related to the varicella-zoster virus, the same virus which causes chicken pox. Instead of leaving your body, the chicken pox hides out in the ganglion cells in the spine. When you are stressed or your immune system is weakened in some other way, it can come out and cause an extremely painful rash, typically on one side of your body in a stripe.
Much more painful. Shingles (herpes zoster) is a reactivation of the varicella virus which causes the chicken pox many adults had as children. It causes an exquisitely painful rash which occurs along usually the trunk. Shingles often occurs in straight lined clusters called dermatomes corresponded to the nerve the virus infects. It is treatable with antiviral medications if caught early.