A cluster is the smallest single unit of the space on a disk (a hard disk or a floppy or even an optical disc) that your computer's operating system keeps track of separately. The operating system keeps systematic records of which clusters are occupied by each file stored on the disk (in DOS, this is called the file allocation table, or FAT). Clusters usually consist of more than one sector, a sector being the smallest unit of disk space that the computer can read data from or write data on. There are too many sectors on a hard disk to keep track of them all individually, so the operating system deals with them in groups called clusters instead.
Sometimes the operating system gets confused so it thinks a cluster is in use even though no particular file is using it. This is a lost cluster. You can free up the space occupied by a lost cluster, and maybe get back some missing data, by running a special disk repair utility (such as CHKDSK on the pc).
A group of computers connected together using a very fast NETWORK, with an operating system that enables them to be treated as a single MULTIPROCESSOR computer or to provide FAULT-TOLERANT backup for one another. Running tasks may be automatically switched from one member of the cluster to another in the event of its failure: