All modern hard disks contain an integrated cache, also often called a buffer. The purpose of this cache is not dissimilar to other caches used in the PC, even though it is not normally thought of as part of the regular PC cache hierarchy. The function of cache is to act as a buffer between a relatively fast device and a relatively slow one. For hard disks, the cache is used to hold the results of recent reads from the disk, and also to "pre-fetch" information that is likely to be requested in the near future, for example, the sector or sectors immediately after the one just requested.
The use of cache improves performance of any hard disk, by reducing the number of physical accesses to the disk on repeated reads and allowing data to stream from the disk uninterrupted when the bus is busy. Most modern hard disks have between 512 KiB and 2 MiB of internal cache memory, although some high-performance SCSI drives have as much as 16 MiB, more than many whole PCs have!