Cloud storage employs software that interconnects and facilitates collaboration between different types of storage devices. Compared with traditional storage methods, cloud storage poses new challenges in data security, reliability, and management. This paper introduces four layers of cloud storage architecture: data storage layer (connecting multiple storage components), data management layer (providing common support technology for multiple services), data service layer (sustaining multiple storage applications), and user access layer. A typical cloud storage application—Backup Cloud (B-Cloud)—is examined and its software architecture, characteristics, and main research areas are discussed.