Answer:
    1. Minhag. Custom. Custom, although not really part of Halachah, can
       change. Minhag is any act that the masses, on their own, accept.
       Any minhag that is against actual Halachah, is called a minhag
       ta'os, a mistaken minhag. Any that is based on a misunderstanding
       is a minhag shtus, a foolish custom. These two should not be
       followed. Any nearly universal minhag is called a Minhag Yisroel,
       and has most of the stringencies of law. (Yarmulka, and Ma'ariv
       services are two examples of a Minhag Yisroel.)
    2. Din dirabanan. A rabbinic law. These are set up by the rabbinate,
       instead of the masses, in order to preserve the spirit of the law.
       For example, Purim and Chanukah. There are 7 new commandments that
       are entirely rabbinic, bringing the famous total of 613 mitzvot up
       to 620.
    3. Gezeira dirabanan. A rabbinic "fence". These are enacted to
       prevent a common cause for breaking the act of the law. For
       example, one may not place food directly on a fire before Shabbat
       in order to keep it heated during Shabbat. This is a fence around
       the law against cooking on Shabbat. To prevent the gezeira from
       being violated, a metal cover, called a blech in Yiddish, is
       placed on the stove top before Shabbat with the flame (turned to a
       low setting) under one section and the pot with food placed on the
       blech. This blech serves as a fence, allowing heating of the food
       without any danger of violating the law. Note that a "gezeira
       dirabanan" becomes binding only if it is accepted by the
       community.
    4. P'sak. A rabbinic ruling. This ruling addresses a the questionable
       area of some law or custom. A p'sak can only be over ruled by
       another body which is both larger in number, and greater in
       "chochmah". (The ability to know how to use the facts. Not more
       knowledgeable book-wise, but more steeped in the Torah
       weltanschauung.)
       
   The distinction between the second and third categories is subtle. In
   order to be a Din (or Issur, or Melachah) Dirabanan, the prohibited
   action must be similar in purpose to the permitted one. A gezeira does
   not even require an action. In the example I gave, it was inaction,
   leaving the pot where it is, that is prohibited. The category includes
   things that are similar in means to the prohibited act, and will
   therefore cause confusion about what is and what isn't okay; and
   things which will allow people to be caught up in habit, and forget
   about the prohibition. Only a gezeira may defy an actual Divine law
   (although a p'sak will often define one), and even so only under
   specific circumstances. All of the following must be satisfied:
     * The law being protected is more stringent than the one being
       violated. This determination isn't easy.
     * The law is being violated only through inaction. No one is being
       told to actively violate G-d's commandment.
     * The law being violated will still be applicable in most
       situations. It still must exist in some form.
       
   On the other hand, a gezeira is less powerful than a normal rabbinic
   law in that they can not be compounded. One may not make a "fence" for
   the express purpose of protecting another "fence". A law is considered
   accepted if it becomes common practice. Any din or gezeira which does
   not get accepted by the masses in the short run, does not become
   binding in the long run. Similarly, there are rules for p'sak, but
   they are violated if the masses choose to follows some other rabbinic
   body's p'sak. Notice, however, that this is only in the short run.
   Once a law is accepted, it may only be overruled by p'sak. It cannot
   just fade into non-practice.