Magnolia can be configured to run in a clustered environment to provide high availability and load balancing.
High-availability clusters are also known as fail-over clusters. Their purpose is to ensure that content is served at all times. They operate by having redundant instances which are used to provide service when a public instance fails. The most common size for a high-availability cluster is two public instances, the standard Magnolia setup. In such a setup the redundant instance may even be dormant (not actively serving content) until it is called to service.
Load-balancing clusters connect many instances together to share the workload. From a technical standpoint there are multiple instances but from the website visitor's perspective they function as a single virtual instance. A load balancer distributes requests from visitors to instances in the cluster. The result is a balanced computational workload among different instances, improving the performance of the site as a whole.