What is the effect of the "s" permission bit on a file?

I have two files with the following permissions:

-rwsr--r--  1 root root 213 Oct 22 12:15 f1
-r--rwxr--  1 Bob staff 113 Oct 22 12:18 f4

Can the user Bob execute f1 and why? What is the effect of the s in the set of permission on f1?