It is usually suggested that Shakespeare's plot line follows Freytag's Pyramid structure. This five part structure creates a visual of rising and falling action that helps the reader see how the parts of the plot relate to the overall intention of the play. The first piece of the structure is the EXPOSITION. In the early part of the play the basic situation and premise(s) are presented, the the initial conflicts are established, and the the major characters are introduced. This is usually done within what we know to be Act 1.
The next piece of the structure is the RISING ACTION. It is here that the plot becomes more complicated. There are more characters introduced, more sub-points of the conflict arise, and the story starts to pick up the pace. The third part of the plot is the CLIMAX of the play. This is not necessarily the most interesting thing that happens, but climax is better defined as "the point from which nothing can go back to the way it was before." In most plays, it isn't an action, but a decision made by the main character.
In Hamlet it could be his decision to not kill Claudius when he had a chance. In Romeo and Julietit could be Romeo's decision to avenge Mercutio's death -- an act in direct violation the Prince's command and the new-found family relationship he has with the Capulet family.
The fourth part of the play is the FALLING ACTION. This is the immediate aftermath of the decision made at the climax of the play. The actions of all of the characters are speeding toward the inevitable end of the play. The RESOLUTION is the final part of the play, and it is here that the audience is most invested in the results of the play. These are usually the moments of greatest interest,all of the subplots come together, and all of the conflicts come their conclusion. This structure is rather predictable, and can help the reader know what to expect at each section of the play.
It is important to note too that while Shakespeare's plays are all divided into five acts, the acts don't always match up perfectly with the pyramid and that Shakespeare didn't make the act divisions; that it the work of the writers, editors, and printers of the Folios and Quartos who were trying to print complete scripts of the plays after the writing and productions were done by Shakespeare's acting companies.