Just a quick vocabulary lesson today!
There are many ways to classify mitzvot, and one of the major ways is by source: from the Torah or from the rabbis.
Mitzvot d'oraita (mitzvos d'oraisa) are mitzvot directly from the Torah, both the Written Law and the Oral Law. The theoretical punishment (when there was/will be a Sanhedrin) for breaking these mitzvot is stricter than those d'rabbanan. Generally, there are less leniencies allowed, so it's sometimes important to know whether a mitzvah is d'oraisa or d'rabbanan.
As you can probably guess, mitzvot d'rabbanan (mitzvos d'rabbanan) are derived from rabbinic sources. There can be a much greater variation of observance with mitzvot d'rabbanan than mitzvot d'oraisa. This is where you find the rabbinic "fences" that are intended to prevent people from accidentally violating mitzvot d'oraisa. The rabbis can and do still institute new mitzvot. It's totally dated by now, but I always think of when electricity and cars were invented.
Related to this discussion are minhagim (plural of minhag, custom). Customs are considered a subset of mitzvot d'rabbanan because they are adopted by the rabbis as mitzvot, but they are less clearly connected to the Torah.
If you want to read slightly more in depth on the subject, check out this good beginner's reference at Jew FAQ!