Wikipedia
is the command in the Linux operating system that allows a user to set certain attributes of a file residing on a Linux file system. is the command that displays the attributes of a file.
Modern BSD-like systems, including OS X, have analogous to set, but no command specifically meant to display them; specific options to the command are used instead.
The Solaris system has no commands specifically meant to manipulate them. and are used instead.
Other Unices, in general, have no analogous commands. The similar-sounding commands (from HP-UX) and (from AIX) exist but have unrelated functions.
Among other things, the command is useful to make files immutable so that password files and certain system files cannot be erased during software upgrades.