Clustering - A technology to solve common computational problems
The first time I heard of a cluster was in my schooling days when my science teacher talked about Star clusters to denote a group of stars. Clustering is the process of organising objects into groups whose members are similar in some way.
In the computer world, when people talk about clustering, they are refering to technologies that allow multiple computers to work together to solve common computing problems.
Clusters are also used in normal computing areas in Linux where a degree of redundancy is required. For example, if you are running a database server, then you can make it fail safe by using a database cluster.