The following taxonomy of kinds of halachic ruling was culled from the Rambam, Hilkhos Mamrim ch. 2, and includes thoughts learned at a shi’ur given by R’ Yonasan Sachs (of RIETS and the Agudath Israel of Passaic).
(I posted an earlier version on soc.culture.jewish, and from there made it into the group’s FAQ. I later repeated it on Avodah, and the following reflects comments and corrections made there.)

1. Minhag. Custom. Custom, although not really part of halakhah, can change. Minhag is any act that the masses, on their own, accept. According to the Rambam, to qualify as a minhag the practice must then ratified by the rabbinate. Any minhag that is against actual halakhah, is called a minhag ta’os, a mistaken minhag. Any that is based on a misunderstanding is a minhag shetus, a foolish custom. These two subtypes should not be followed. Any nearly universal minhag is called a minhag Yisrael, and has most of the stringencies of law. Yarmulka and ma’ariv services are two examples of a minhag Yisrael.
2. Din deRabanan. 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. According to the Rambam, who only counts biblical mitzvos amongst the 613, this means there are actually 620 mitzvos altogether.
3. Gezeira deRabanan. 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 Shabbos in order to keep it heated during Shabbos. This is a fence around the law against cooking on Shabbos. To prevent the gezeira from being violated, a metal cover, called a “blech” in Yiddish, is placed on the stove top before Shabbos 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 the community accepts it.According to the Rambam, a gezeira cannot be overturned. However, a gezeirah where the law’s purpose is included in the legislation is implicitly conditional on the purpose. The problem is in knowing when the purpose is given in the quoted gezeirah, and when the gemara provides a motivation on its own, after quoting the gezeirah. For example, meat must be salted within three days of slaughter, or the prohibited blood will be too soaked into the meat to be retrieved. What about the contemporary situation, where meat is generally frozen solid? Some rule that since the reason is given in the legislation, and the reason doesn’t apply, neither does the time limit. Others rule stringently, presumably because they do not believe the reasoning about the blood being soaked into the meat was part of the legislation as initially codified.

   According to the Tif’eres Yisrael (Ediyos 1), there are actually two sub-categories:

   1. Siyag. Fence (Hebrew; “gezeirah” is Aramaic). Something that will lead to a future violation to do an error in understanding the law. Such as the ban on mixing poultry and milk, lest people become lenient in mixing meat and milk.
   2. Cheshash. Concern. Cases where the threat of violation is in the current situation, because one is in a circumstance where habit taking over or other accident is likely.

   The Tif’eres Yisrael says that a cheshash can be deemed inapplicable if the norms change such that the threat no longer exists. It does not require a beis din that is greater in number or wisdom as the law is not lifted, just that the current situation is deemed to be outside the limits the law addressed.

4. Asmachta. Mnemonic. The Raavad (on Mamrim 2) considers laws backed by a mnemonic in the Torah are in a different category than other rabbinic laws. He writes that Hashem wrote these asmachtos as a way to suggest laws to us to enact as needed.
5. Divrei Qabbalah. The words that were received; i.e. laws enacted by a beis din like the Great Assembly that included nevi’im (prophets). Many consider these rabbinic laws one step closer to Torahitic law than most others, since the law was ratified by consulting with Hashem Himself. This is like the Raavad’s concept of asmachta, but more so — not only suggested by Hashem to be used as needed, but we’re told that the situation justified it.Those who believe this is a distinct category would include Purim as divrei Soferim rather than usual rabbinic law. Some achronim rule that the obligation for women must hear megillah and the other mitzvos of Purim is rabbinic but for men it’s divrei qabbalah. Thus, their obligation is lesser.
6. Pesaq. A rabbinic ruling. This ruling addresses a the questionable area of some law or custom. A pesaq that is not prima facie in violation of accepted halakhah can only be overruled by another body that is both larger in number (or perhaps number of students), and greater in “chokhmah”. (The ability to know how to use the facts. Not more knowledgeable book-wise, but more steeped in the Torah weltanschauung.)
7. Derashah. A law derived by hermaneutics. Some hold that derashah only serves as support for already known laws, and therefore are tools in pesaq. However others, including the Rambam, see them as constructive, a means for discovering new Torahitic laws. This appears to be supported by a medrash on Rus, in which Boaz is credited as being the first to rule that “Moavi” means males from Moav in particular.
8. A last category has only two related examples. Torah law mandates a shevus, a law to rest on Shabbos. It also requires resting from some of the melachos, constructive work activities, even on chol hamo’ed. However, Hashem left it up to man to decide the parameters of these forms of rest.

The distinction between the second and third categories is subtle. In order to be a din (or issur, or melakhah) deRabanan, the prohibited action is one that is similar in purpose to the permitted one.

In contrast, 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. Second, 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 pesaq 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.
• According to the Ta”z, the law being violated will still be applicable in most situations. It still must exist in some form. (Not every acharon agrees with this requirement.)

In another way, a gezeira is less powerful than a normal rabbinic law in that it cannot 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 that does not get accepted by the masses in the short run, does not become binding in the long run. Similarly, there are rules for pesaq, but they are violated if the masses choose to follows some other rabbinic body’s pesaq. Notice, however, that this need for acceptance is only in the short run, to enact the law. Once a law is accepted, it may only be overruled by pesaq. It does not cease to exist just because it faded out of practice.