Unix is a family of multiuser, multitasking operating systems found on many servers, workstations, network equipment and other embedded devices. Unix variants include linux, macosx, openbsd, solaris, and more. Use the tag unix for questions that are about Unix-like systems in general; if your question is about a specific operating system (Linux, OpenBSD, …), use the tag for that system. For functional questions (“How do I use this tool which is not specifically a security tool?”), try asking on our sister site Unix & Linux.
Resources such as data files, programs and devices are accessed through files. In the traditional Unix model, each file has an owner and group, and the owner, group and others are granted separate permissions (read/write/execute). Many modern Unix systems provide access control lists and other extensions to the basic model.