su on its own just does the job of substituting your current user login with a shell of another user. For any non-root user this will require a password to be entered for the destination user.
sudo su means that someone has created a sudo rule allowing a certain user to run a command with escalated privileges. So you might grant a certain person sudo rights to run su to allow them to become root or any other user they chose, possibly without entering a password (depending on the rule). If it does ask for a password, it will be the password of the user granted the sudo right, not the destination user.
Annihilannic.