Filesystem ACLs
A filesystem ACL is a data structure (usually a table) containing entries that specify individual user or group rights to specific system objects such as programs, processes, or files. These entries are known as access control entries (ACEs) in the Microsoft Windows NT, OpenVMS, Unix-like, and Mac OS X operating systems. Each accessible object contains an identifier to its ACL. The privileges or permissions determine specific access rights, such as whether a user can read from, write to, or execute an object. In some implementations, an ACE can control whether or not a user, or group of users, may alter the ACL on an object.
Most of the Unix and Unix-like operating systems (e.g. Linux, BSD, or Solaris) support POSIX.1e ACLs, based on an early POSIX draft that was abandoned. Many of them, for example AIX, FreeBSD, Mac OS X beginning with version 10.4 ("Tiger"), or Solaris with ZFS filesystem, support NFSv4 ACLs, which are part of the NFSv4 standard. There are two experimental implementations of NFSv4 ACLs for Linux: NFSv4 ACLs support for Ext3 filesystem and recent Richacls, which brings NFSv4 ACLs support for Ext4 filesystem.
Read more about this topic: Access Control List