OS X supports two kinds of file and folder permissions:
Standard Portable Operating System Interface (POSIX) permissions
Access control lists (ACLs)
Standard POSIX permissions let you control access to files and folders based on three categories of users: Owner, Group, and Others. Although these permissions give you some control over who can access a file or a folder, they lack the flexibility and granularity that many organizations require in dealing with complex user environments.
This is where ACLs come in handy. An ACL provides an extended set of permissions for a file or folder, and lets you set multiple users and groups as owners.
For Active Directory user accounts, ACLs for files on OS X computers bound to AD are compatible with Windows ACLs, giving you added flexibility in a multiplatform environment.