A file system (or filesystem) is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device(s) which contain it.
A file system organizes data in an efficient manner and is tuned to the specific characteristics of the device. There is usually a tight coupling between the operating system and the file system. Some filesystems provide mechanisms to control access to the data and metadata.
Ensuring reliability is a major responsibility of a filesystem. Some filesystems provide a means for multiple programs to update data in the same file at nearly the same time.