From sheep101.info:


The primary difference between ruminants (eg., cows) and simple-stomached animals, like people, dogs or pigs, is the presence of a four-compartment stomach that includes the rumen, reticulum, omasum, and abomasum. Often, it is said that ruminants have "four" stomachs. Llamas and alpacas are "pseudo-ruminants" because they have a three-compartment stomach instead of four like ruminants. Horses are also not ruminants; however they have a "cecum" that performs a similar function as the cow or sheep's rumen.