In the computer world, a cluster can refer to two different things:
1) A group of sectors on a disk. While a sector is the smallest unit that can be accessed on your hard disk, a cluster is a slightly larger unit that is used to organize and identify files on the disk. Most files take up several clusters of disk space.
Each cluster has a unique ID, which enables the hard drive to locate all the clusters on the disk. After reading and writing many files to a disk, some clusters may remain labeled as being used even though they do not contain any data. These are called "lost clusters" and can be fixed using ScanDisk on Windows or the Disk Utility program on the Mac. This is why running a disk utility or defragmentation program may free up space on your hard disk.
2) A group of connected computers. A cluster can also refer to several machines grouped together, all performing a similar function. For example, a cluster may consist of eight PCs, all connected via high-speed Ethernet, processing scientific data. This type of setup is often referred to as "parallel computing," since all the computers in the cluster are acting as one machine. Clusters are typically used for high-end processing, such as performing scientific calculations or decrypting algorithms.