Because this play is a re-telling, so to speak, of the Salem Witch Trials, each act presents one stage or period of the trials.
The dramatic action of Act I is watching the hysteria and panic rise to a level which finally causes the first "calling out" of witches in Salem. The act ends with the frightened, guilty, and impressionable girls screaming out the names of women in the town--the first "hit list," if you will for the subsequent trial. This act employs dramatic irony, in the sense that the audience knows what silliness prompts this outcry, but most of the characters do not.
Act II depicts the actual trials, events full of dramatic moments. We get a clearer indication of Abigail's motives; a more complete understanding of Proctor's guilt, shame, and resolve to repair the damage he has caused; a closer look at the accusation and arrest process for those who have been "named"; a picture of a husband and wife who love each other but end up in jail for that very reason; and a town and court system gone utterly mad with witch hysteria.
Finally, Act III shows both a picture of redemption and the tragic conclusion to this ugly episode in history. Proctor, a self-accused sinner, cannot, in the end, confess falsely to the one sin he did not commit; in doing so, he regains his soul and finds some peace for his guilt-ridden conscience. As these innocent people die on the gallows (in front of a crowd which has finally lost its taste for hangings), we understand this awful injustice is the beginning of the end.
As a whole, the play presents in several hours what, in reality, took several years. Each act creates dramatic impact for each aspect of the historical event.