First the null list. As you know sending a message back to a potential spammer will often not work. Many times their return addresses are fake or their mail boxes are full. The spammers ISP will often send a message back to you to tell you that the email address is invalid or the user's mail box is full. You don't care about these messages and they would only create more spam, though not commerical junk email - unwanted email nonetheless. So you add email addresses to your null list to tell MAPS to just discard these messages. Entries in your null list can be generic in nature. For example, "mail-daemon" is a common string in a bounced email address from an ISP. Entering "mail-daemon" will discard messages from mail-daemon@aol.com as well as mail-daemon@msn.com. Most users do not care to receive email messages from mailer daemons.
The black list is for people who you are effectively ignoring. It works similarly to null lists except that a message telling the sender that you are ignoring them is returned to the sender. Use this when you want to make sure the sender knows that you are ignoring them.
The white list is the list of people who are allowed to email you. Management of the white list is up to the people who wish to email you, thus you are not bothered by the hassle of having to maintain a white list. You can manage your white list, pre-registering a user or say initializing your white list from your addressbook (called Seeding your White List). But by and large the white list will be managed by your legitimate email subscribers.