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File control blocks (FCB), sometimes referred to as file descriptors, are data structures that hold information about a file. When an operating system needs to access a file, it creates an associated file control block to manage the file. The structure of the file control block differs between operating systems, but most file control blocks include the following parts:

Filename of Directory name
Location of file on secondary storage
Length of file (usually in bytes)
Date and time or creation or last access

Access Rights (Read only, Hidden, Write, etc)

When the operating system is first installed, it creates a root file system on the disk that specifies how many sectors are available and how they will be allocated. The root file system is a table of entries like a directory. In general, this is a fixed size, and once full, no more entries can be added.  Each entry can be either a file or another directory table. The following table depicts this structure

To make things a little simpler Rather than managing a large number of sectors, the operating system groups sectors together into a minimum allocation unit called a cluster. When a request to create a file occurs, the operating system allocates a cluster at a time until the all the data is stored. This raises a question.

The operating system can maintain a table of cluster entries, and mark each cluster as either free or allocated. This was a technique used in the MS-DOS operating system. Other operating systems maintain a linked list of free clusters, each free cluster pointing to the next free cluster. As clusters are allocated, they are removed from the free cluster list. When a file is deleted, the clusters that were allocated to it are added back to the free cluster list.

In multi-user operating systems, files may be accessed by multiple users. Permission rights associated with folders (directories) and files are used to protect or restrict access to files. In UNIX these rights are known as Read, Write and Execute. In Windows NT and Windows 2000 (using the NTFS file-system only as permissions are not supported with FAT), additional file permissions are available. In UNIX, three groups of permissions apply to every file. The first group of permissions defines those of the owner of the file, and are a combination of the read, write and execute permissions. The second group of permissions defines those permissions for a group of users. The third group of permissions defines the permissions for everyone else except the owner and group members.