This is a green black deck that is focused on the "Undergrowth" ability which does things based on the number of creatures in your graveyard. Stuff like: "When this enters the battlefield get a +1/+1 counter for each creature in your graveyard". It can be anything though, it depends on the card: Gaining life, gaining temporary buffs, counters, etc. Golgari does not mind it's creatures dying, it uses that as an advantage.

In the next set, Ravnica Allegiance, this pairs well with:
Orzhov (Black / White)
Rakdos (Black / Red)
Gruul (Green / Red)
Simic (Green / Blue)