It's called suid bit or sgid bit by default, unix installations have many programs installed suid root. Suid root refers to set user id root. This allows the program to do functions not normally allowed for users to do themselves. Low level networking routines, controlling graphical display functions, changing passwords, and logging in are all examples of programs that rely on executing their functions as a user that is not restricted by standard file permissions. While many programs need this functionality, the program must be bug free in only allowing the user to do the function the program was designed for.
DynaMite
