All Greek deities can be divided into two types: the Olympians, who were thought of as living on Mount Olympus, or somewhere in the heavens, and the Chthonians, who were thought of as living below the earth. Their name comes from chthōn (χθών), which is the Greek word for “earth” or “ground”. In order to sacrifice to one of the Olympian gods, an animal would be ritually slaughtered and the less edible pieces would be burned on an altar so the smoke could waft up to the Olympians and nourish them. In a sacrifice to one of the Chthonians, on the other hand, a pit would be dug in the ground and an animal would be slaughtered (without any burning) in such a way that the blood would pour into the pit. Libations (such as milk or wine) could also be poured into the pit to give the gods sustenance. For a good example of chthonian sacrifice, see Odyssey Book 10 lines 515-530 (in the Greek) or Odyssey Book 11 lines 23-37 (in the Greek).
Mythology Unbound: An Online Textbook for Classical Mythology by Jessica Mellenthin and Susan O. Shapiro is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.