What is a Reminder in GTD?
It is the representation of anything you consider incomplete in your world. In essence, they are what your lists are made of, and so the key substantive thing to organize.
Certain kinds of input will most efficiently serve as their own reminders of required actions, rather than you having to write something about them on a list. This is particularly true for some paper-based materials and some e-mails, as well as some things that you need to read or review.