Windows 2000 professional User Groups

Ok, I am wondering, what is the SYSTEM user group?? I have 2 user Groups I use: Administrators and Users. I am currently planning on using the security templates in mmc. I only want to use 3 users: Administrator, User (part of the user group), and SYSTEM. do you think i am doing the right thing?? i am going to use the setup security template as the security configuration.