The chown command (abbreviation for change owner) is used on Unix-like systems to change the owner of a file system object (files and directories). Unprivileged (regular) users who wish to change the group of a file that they own may use chgrp.
From the BSD man page for chown:
For obvious security reasons, the ownership of a file may only be altered by a super-user. Similarly, only a member of a group can change a file's group ID to that group.