When You Pretend To Be Someone Else

executables may be set so that when user X executes them, that process runs with the privileges of the owner of the file. This is known as 'SUID' programs.

The owner of an executable may turn it into an SUID program using the 'chmod u+s <filename>' command.

A file system may be mounted with the 'nosuid' option, and then the 's' bit on files residing under it will be ignored by the system.