Play Collar - This type of collar is used only during play sessions, be it to put the submissive into a submissive mind set, or to attach to a leash.
Training Collar - This type of collar indicates that the submissive is taken, and is in training for being permanently collared. It can also be used while a dominant is mentoring a submissive.
Permanent/Slave Collar - This type of collar is as important, if not more so than a marriage band, and is worn constantly.
Protective Collar - This type of collar is given to a submissive by a dominant to indicate that she is under his protection, even though she is not actually his.