• Resource outsourcing. At least some computing resources are controlled by someone else, and those resources live somewhere else. Infrastructure, operations, and physical security are a concern.
• Multi-tenancy. While not a strict prerequisite for the cloud by most people's definition, it's still one of the things that makes many cloud deployments special.
• Scale. Because cloud applications often host many tenants, they sometimes scale far beyond what traditional applications require in terms of data and request rates, depending on the technologies under the hood.