For those who don't have English as their mother language, it can be surprising to discover that the word 'cluster' is actually a noun and a verb; it means a group of similar objects growing closely together (noun) and, also, congregating or being part of a group (verb). Of course, nowadays, the term is popular as computer clusters have become widespread.
Based on that, WebLogic Server cluster or clusters basically means that a group of servers will work together, running at the same time, toward some predefined architectural goals. Clusters can be created to cater to different needs of users, such as scalability, reliability, performance, load balancing, failover, and management. Note that WebLogic clusters can be defined by multiple machines and servers (Managed Servers) but they may also be seen as one single WebLogic Server for applications and clients.
Clustering is a key concept in application scalability and needs to be used in order to scale out (horizontally...