In Maus, Jews are portrayed as mice, while Germans are portrayed as cats. The metaphor of Jews as mice is taken directly from Nazi propaganda, which portrayed the Jews as a kind of vermin to be exterminated. The cat/mouse relationship is also an apt metaphor for the relationship between the Nazis and Jews: the Nazis toyed with the Jews before ultimately killing them.
The decision to portray different races as different kinds of animals has been criticized as over-simplistic and for promoting ethnic stereotypes. Beneath the simple metaphor, however, is an earnest attempt to illustrate the unyielding stratification by class and race that was very much a part of life in World War II-era Poland.