|PolicyKit Library Reference Manual|
Traditionally UNIX-like operating systems have a clear distinction between ordinary unprivileged users and the almight and powerful super user 'root'. However, in order for a user to access and configure hardware additional privileges and rights are needed. Hitherto, this have been done in a number of often OS-specific ways. For example, Red Hat based systems usually grant access to devices to a user if, and only if, the user is logged in at a local console. In contrast, Debian-based systems often relies on group membership, e.g. users in the 'cdrom' group can access optical drives, users in the 'plugdev' group can mount removable media and so on.
In addition, access was not only granted to devices; Red Hat-based systems, for example, provides a mechanism to allow a user at a local system to run certain applications (such as the system-config-* family) as the super user provided they could authenticate as the super user (typically by entering the root password using a graphical utility). Other distributions rely on sudo (with various graphical frontends) to provide similar functionality. Both the pam-console and sudo approaches doesn't require applications to be modified.
Finally, some classes of software (such as HAL, NetworkManager and gnome-system-tools) utilizes IPC mechanism (typically D-Bus) to provide a very narrow and well-defined subset of privileged operations to unprivileged desktop applications. It varies what mechanism is used to deny users.