A computer cluster is a group of linked computers, working together closely thus in many respects forming a single computer.
Cluster and Cloud computing would fall under distributed computing Both use large number of servers but the difference lies in how the process and resources are shared and executed.
Cluster computing refers to many/group of computer which are linked together and they work together on a process. We use cluster computing to improve performance and availability when compared to a single stand alone computer. Cluster computing is a part of distributed computing as JamesZach said above. A common example which we see of cluster computing is for load balancing servers where the load is distributed equally among clusters.
While it has it differences, it's mostly used to describe the same thing. Linking many computers or instances together.
It is a type of network relating with computer linked together where documents and assigned work shared and executed.
Several computers are linked together by communication medium and work as a single computer. It is one kind of distributed system.