xsu is an interface for the command `su - username -c command` in Gnome. When the user executes xsu, he/she will be prompted for a command, a username and a password. If the username and password are correct xsu will execute the command as the entered user. So if the username was root, then the command will be executed as root.
Of course it's also possible to predefine the username and the command by using the [-u,--username] and [-c,--command] arguments at the command line of xsu. This can be very useful when using xsu in for example gmenu or the GNOME panel.
* Security Gnome Xsu 2.0 uses the standard su binary to gain it's root access. This way, all security issues should be solved.
The security of all older xsu versions was very bad. With xsu version 0.1.0 the use of a pipe to a secured non-Gnome SUID application was introduced. This has been replaced with a special type of pipe to the standard GNU su application from the 'login' package.