Linux - What s means in the permissions mask of a file?

Question

If I run
ll /usr/bin/crontab

it gives me this output:
-rwsr-sr-x 1 root root 315432 Nov 10 2007 /usr/bin/crontab

What does 's' means in the permission mask?

Why do they need to use that?

Thanks, Dan

Answer 1

The s means setuid (or setgid). In this case the crontab program will run with root permissions (because root is the owner) when ran by other users.

Answer 2

These are permissions as listed with the ls command.

r=read

w=write

x=executeable

s=setuid