This document discusses DNA viruses and viral vectors. It begins by introducing viruses and their classification based on host, morphology, genome, and structures. It then describes the Baltimore classification system for viruses. Viral vectors are discussed as effective means of gene transfer that can be manipulated to express therapeutic genes. Several virus types being investigated for gene delivery are described, including retroviruses, adenoviruses, adeno-associated viruses, and herpes simplex viruses. Key properties of viral vectors like safety, toxicity, stability, and cell specificity are outlined. Specific types of viral vectors - retroviral, lentiviral, adenoviral, adeno-associated viral, and herpes simplex viral vectors - are then characterized.