Two more types of permission are “set user ID,” known as suid, and “set group ID,” or sgid. These settings, when used in a program, enable any user running that program to have program owner or group owner permissions for that program. These settings enable the program to be run effectively by anyone, without requiring that each user’s permissions be altered to include specific permissions for that program.
One commonly used program with suid permissions is the
matthew@seymour:~$ ls -l /usr/bin/passwd-rwsr-xr-x 1 root root 42856 2010-01-26 10:09 /usr/bin/passwd
This setting allows normal users to execute the command (as root) to make changes to a root-only-accessible file ...