First of all, if you've read or are reading the other reviews, ignore the blather about how the whole "Animal Farm" metaphor--Jews as mice, Germans as cats, etc..--being racist and demeaning.
Art Spiegelman attempts to tell the story of his father Vladek's life in Hitler's Europe. By and large, the book is a detailed, objective retelling of his Vladek's story. However, as Art himself will realize, "I can't even make sense out of my relationship with my father--how am I supposed to make sense out of the Holocaust?" and "Reality is much too complex for comics--so much has to be left out or distorted." Thus liberated from the impossible standard of complete objectivity, Art is free to insert two important subjective elements into the story--the depiction of different races as different species, and the insertion of himself as a character in MAUS.
Obviously, Art is not a overt racist--in fact, in the second part of MAUS, Art will scold his father for distrusting a black person, and a German-Jewish couple will help Vladek return home after being freed from the death camps. The point of portraying Jews as mice, Germans as cats, Poles as pigs, etc. is to show what race relations during Hitler's Europe might have been like.
The characterization of race doesn't end there, though--as the scene shifts from Nazi Germany to the present, and as Art must suffer the daily trials and tribulations of life with a father permanently scarred by his experiences, Art depicts himself as a mouse as well, a confession that he himself is unable to completely escape the aftermath of the poisoned race relations of the Holocaust. Maybe this makes him a covert racist. But if he is, then who isn't?
Art's involvement in MAUS goes beyond interviewing his father, though. Later in the story we will see that Art was treated in a mental hospital and sees a psychiatrist regularly. As the book cover declares, "MAUS is a story about the survivors of the Holocaust--and of the children who somehow survive the survivors."
The storytelling in MAUS is stellar, and the craftsmanship is as well. The comics medium allows Spiegelman to employ some interesting tricks. For example, whenever Vladek is trying to sneak around, he is portrayed with a pig mask. When Vladek and Anja are trying to escape from the ghetto, Anja, who in real life was easily identifiable as a Jew by her appearance, is drawn with a long tail, while Vladek is not.
In sum, MAUS is a gripping story of his parents' experience during the Holocaust, filled with countless brushes with death, tales of betrayal, and plenty of terrible, graphic illustrations of victims being executed. It is not a history text in the most austere and empirical sense. Rather, it is a confession that the Holocaust defies dispassionate and detached analysis.