In computing, a cluster may refer to two different things: 1) a group of sectors in a storage device, or 2) a group of connected computers.
1) A group of sectors
A sector is the smallest unit that can be accessed on a storage device like an HDD or SSD. A cluster, or allocation unit, is a group of sectors that make up the smallest unit of disk allocation for a file within a file system. In other words, a file system's cluster size is the smallest amount of space a file can take up on a computer.
Most files require a large number of clusters, which means the file contents are spread across multiple clusters on the drive. Often the data can be written in contiguous blocks so that the file contents are stored in one physical location. However, when a hard drive begins to fill up, there may not be enough contiguous clusters available to save large files in a single area. Instead, they must be written in multiple locations on the disk. This is called fragmentation and can slow down the hard drive's read and write speeds.
2) A group of connected computers
A cluster can also refer to a group of machines that work together that perform a similar function. Unlike grid computing, a computer cluster is controlled by a single software program that manages all the computers or "nodes" within the cluster. The nodes work together to complete a single task. This process is called "parallel computing" since the nodes perform operations in tandem.
Computer clusters can range from two machines to hundreds of connected computers. Small clusters are often used to improve the performance of web and online gaming services by handling multiple incoming requests in parallel. A web farm, for example, is a type of cluster that provides low latency access to websites. Large clusters can be used to perform scientific calculations or to run a large number of complex algorithms. For example, a large cluster may be used to apply textures and lighting effects to 3D models in each frame of an animated movie.