Answer

As far as I know, these categories really are somewhat vague and not fully defined. I’ll write what seems right to me. You can look in the topics about utensils at the beginning of chapters 1 and 2 of Maimonides’ Laws of Rebels.
Rabbinic commandments are commandments that the Sages instituted which have value in and of themselves (like Hanukkah and Purim and others. It is commonly accepted that there are seven such commandments. See Nachmanides in his glosses to the first principle).
Ordinances, simply speaking, are basically the same as rabbinic commandments. One could distinguish by saying that ordinances are meant to fix or regulate something, as opposed to commandments that have intrinsic value.
Decrees are rabbinic laws meant to prevent stumbling in a Torah-level prohibition. Fences would seemingly be the same thing, but perhaps fences are an extension of the Torah prohibition and not necessarily a decree lest one come to violate a Torah law. Similar to what the later authorities (Acharonim) discussed regarding rabbinic Sabbath prohibitions: are they because of a concern that one may come to violate a Torah prohibition, or are they an extension of the Torah prohibition itself? For example, riding a horse is not an extension of reaping, but a decree lest one come to reap. But separating food from waste could also be interpreted as an extension of the forbidden labor of selecting.
In Maimonides another category also appears: customs. In the context of rabbinic laws, this means a custom instituted with the approval of a religious court, even if they did not establish it as one of their own binding ordinances; rather, they gave approval to what the public had already practiced, or they began guiding the public in that direction.
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