Herpes zoster, also known as shingles, is caused by the reactivation of the varicella zoster virus which causes chickenpox. It presents with severe pain and a rash of grouped vesicles in a dermatomal distribution, most commonly on the thoracic region. Complications can include secondary bacterial infection, disseminated lesions, herpes zoster ophthalmicus, and post-herpetic neuralgia. Treatment involves antiviral medications like acyclovir or famciclovir to reduce symptoms and duration.