A group of bytes handled, stored, and accessed as a logical data unit, such as an individual file record.
A group of disk sectors. The smallest allocatable unit of disk storage allowed; each FAT entry represents one cluster. Under FAT16, an average cluster is 16K; under FAT32, clusters are only 4K on partitions up to 8GB.
A sector is a logical segment of information on a particular track, and is the smallest addressable unit of storage on a disk. Tracks are divided into sectors, with each sector 512 bytes long. They contain data, but also contain information as to where the data is located, among other useful bits of information. Modern drives use ZDR - Zone Density Recording, where there are more sectors per track on the outside of the disk where there is more surface area, and fewer and fewer sectors as you go in toward the center of the drive. Newer drives have about 16 zones now. This allows more data to fit on the drive. The outer zones therefore have a higher data transfer rate that those closer to the center of the drive.