The two most popular type of clusters in WebSphere are static clusters and dynamic clusters. Both a static cluster and dynamic cluster are the same in that the cluster is a collection of one or more WebSphere application servers.

However, a dynamic cluster contains features that are not available in a static cluster, which makes a dynamic cluster appealing. 

• Auto start/stop application servers -  This is controlled by the Application Placement Controller.
• Weight and Dynamic workload management (DWLM) - Route traffic to the application server with the most availability

There is more than one way to create a dynamic cluster:

• Create a new dynamic cluster
• Create a static cluster and to then convert the static cluster into a dynamic cluster.

I was kind of surprised when I found that I couldn't add an existing application server to a cluster, and this is what led me into figuring out the different ways that an application server can be part of a dynamic cluster.