In the film, Vito Corleone (Marlon Brando) is the head of a powerful crime family in New York. He arranges killings and is involved in illegal gambling rings, among other things. Yet to those who love him, he is known as a man of reason and justice. He is the Godfather - a title of respect and endearment. Many seek him out for his blessing, his advice or a favor. Despite the fact that he is, essentially, a murderer, the movie does an excellent job of making the audience respect him; the deaths he arranges are always justified in some manner to the audience. They men killed are either other criminals - usually involved in narcotics and other crimes somewhat worse than the Corleones' - or are corrupt in some way, a danger to "the family" and probably murderers themselves.
Vito's family has a hierarchical power structure. At the top is the first-born, Santino "Sonny" Corleone (James Caan), hot-headed and vengeful, with a lust for power - he is inverse of the collected and methodical Vito. Then there is Fredo (John Cazale), the ambitious but ultimately incompetent son who is more interested in women and booze than the family business. Finally there is Michael (Al Pacino) - the youngest son who is clearly the central character throughout the film.
Michael is a young World War II veteran who wants no part in his father's business. His signature line is "That's my family, Kay it's not me." He respects his father more than any other man, but he wants to live a different line. He is a dynamic, fully-realized character that is absolutely convincing. He is also a very different character at the end of the film - but in a way that is completely consistent with his personality.
The film has an excellent pace. There is an ebb and flow - the film begins rather slowly, then picks up the pace, and rises to the climax of the first act - and then the pace slows once more, only to build up suspense again to a final climax, in which all loose ends are neatly tied. Never is the audience bored - the story keeps a tight grip on the viewer, ensuring that they wonder as to the fates of the characters even in the slowest parts - a remarkable accomplishment in of itself.
The Godfather is a excellent film in every sense of the word - technically, aesthetically, in terms of story and characters, themes and music. It immerses the audience in a completely different - yet perhaps altogether to familiar - world. It is a classic that is - and will be - rooted in film history.