Auto-scaling is a feature in cloud computing that automatically adjusts the amount of computational resources based on the server load. Thus, it helps in maintaining high availability and appropriate resource usage without human intervention. Horizontal scaling refers to adding more nodes to a system, which is a strategy used within auto-scaling but is not the feature itself. Disaster recovery is not an active, ongoing scalability strategy, but rather a process enacted after a critical failure occurs. Load balancing refers to the distribution of workloads across multiple computing resources, which, while important for scalability, it does not itself describe the action of scaling resources to match demand.