Orchid2,
References are just a simple letter of recommendation. Most NPC groups require that every potential new member have at least one reference from a member of that sorority. For example, if your Aunt Sue was a member of BBB at State U. and you are going through rush at another campus where there is an BBB chapter, your Aunt could write a letter of reference for you.
You might want to go to the Rush forum and read my
Recruitment FAQ - letters of reference are covered along with other important information about NPC and NPHC recruitment.