I'm in a history seminar class and we're learning about genocides. We most recently covered the Cambodian Genocide in the 1970s. What are the main similarities and differences between the Cambodian genocide and the Holocaust, and what does it reveal about the nature of genocides?

This question is a big one, and in order to conduct a productive analysis, one would have to limit the scope and focus on particular features of genocide.
One of the main differences between the Holocaust and the Cambodian Genocide is the extent to which they have been examined by historians. Study of the Holocaust has given rise to an extremely rich, diverse and sophisticated body of historical literature. (For more information, see the first link below.) For historians, the Cambodian Genocide is recent enough that serious scholarship on the topic is somewhat limited.
It might be helpful to point out some of the features that make the Cambodian Genocide different from most other twentieth-century genocides.
First, the genocide can be seen as anti-colonial or anti-foreign, yet it was perpetrated largely against natives. Cambodia was under a French protectorate from 1863 until 1953 and, during World War II, it had been occupied and suffered at the hands of the Japanese Imperial Army from 1941 to 1945. In 1953, Cambodia became a constitutional monarchy. The Khmer Rouge, the socialist government that took power in 1975 and orchestrated the genocide, was bombed heavily by the United States during the Vietnam War and the civil war in Cambodia, 1967–1975. Resentment of the colonial legacy and foreign hostility contributed to the Khmer Rouge's plan to achieve native self-sufficiency and to eradicate all traces of outside influence. There are some similarities here with the Rwandan Genocide which one might choose to explore in more depth.
Second, the Khmer Rouge, in seeking to eliminate and undo outside influence, was reactionary, even anti-modern. The Khmer Rouge and their leader, Pol Pot, sought to return to a utopian "Year Zero." Generally, twentieth-century states that perpetrated mass violence against their own populations did so in the context of the quest for economic development or modernity (e.g., Stalin-era Soviet famines related to agricultural collectivization and industrialization or Mao's Great Leap Forward famine). The Khmer Rouge brand of socialism was a peculiar one in that it was explicitly anti-modern.
Third, the Cambodian Genocide was unprecedented in its scale in terms of the percentage of the population killed. The Khmer Rouge killed approximately two million, roughly one-quarter, of the country's population. (See the second link below for more statistics.)
In order to draw some productive parallels between the Holocaust and the Cambodian Genocide and from that begin understand the nature of genocide, one might look at the role of nationalism (or ultra-nationalism), radicalism, utopianism, and the settling of perceived scores or righting of perceived wrongs.
One might also begin a comparison by choosing a particular theme or paradigm applied to the Holocaust and then apply it to the Cambodian case. Zygmunt Bauman provided fascinating insights into the nature of modern genocide on the example of the Holocaust. (See the third link below for a primer and possible starting point.)
http://www.soc.duke.edu/~jmoody77/TheoryNotes/Mod_hol_2.htm

https://www.hawaii.edu/powerkills/SOD.CHAP4.HTM

https://www.ushmm.org/collections/bibliography