If you've had chickenpox, the virus that caused it “hides out” in your nervous system. Sometimes this virus reawakens years later and travels to your skin. Lesions can erupt, often on one side of your torso or face. That’s shingles.
In some cases, shingles can cause inflammation of your nerve fibers and roots and damage them, causing pain. You feel the pain in the same areas where the rash broke out. That's postherpetic neuralgia.