Cluster is a collection of something, a simple computer cluster is a group of various computers that are connected with each other through LAN(Local Area Network), the nodes in a cluster share the data, work on the same task and this nodes are good enough to work as a single unit means all of them to work together.
A Hadoop cluster is a special type of computational cluster designed specifically for storing and analyzing huge amounts of unstructured data in a distributed computing environment.
Such clusters run Hadoop's open source distributed processing software on low-cost commodity computers. Typically one machine in the cluster is designated as the NameNode and another machine the as JobTracker; these are the masters. The rest of the machines in the cluster act as both DataNode and TaskTracker; these are the slaves. Hadoop clusters are often referred to as "shared nothing" systems because the only thing that is shared between nodes is the network that connects them.
Hadoop clusters are known for boosting the speed of data analysis applications. They also are highly scalable: If a cluster's processing power is overwhelmed by growing volumes of data, additional cluster nodes can be added to increase throughput. Hadoop clusters also are highly resistant to failure because each piece of data is copied onto other cluster nodes, which ensures that the data is not lost if one node fails.