Why su won't work (su: Authentication failure) ? however sudo work.

Are the permissions correct?

repo@cannabis:~$ ls -l /bin/su
-rwsr-xr-x 1 root root 27908 2009-05-22 17:03 /bin/su
repo@cannabis:~$

su requires you to use the target users password when prompted whereas sudo requires you to use YOUR password when prompted.

That is to say "sudo su - root" would require you to use YOUR password whereas simple "su - root" would require you to use root's password.

sudo is just a tool for granting users access to commands with root permission without actually giving them root. (Though giving someone sudo access to "su - root" means they can do anything they want after they become root including changing the root password.)

In our shop we don't give "sudo - root" but do give things like "sudo su - oracle" to DBAs so they can become the Oracle user to administer Databases. We don't give them access to Oracle direct login because there is no audit log. We don't allow the Oracle user to change its own password.

Are you on Ubuntu? If so,note that the root account is disabled by default. To enable it, enter "sudo passwd root". Once you do this, Ubuntu will behave normally---except that typical GUI admin utilities will still ask for your user password, and not the root password. (I assume this can be changed, but I've never looked into it.)