There are many ways to describe the structure of Othello. The play is typical of Shakespeare is divided into five acts. Act One is the exposition; Act Three is the climax--or turning point of the play, Act Five is the resolution.
But that does not tell you much about the actual plot. You might look at the play as structured around the various characters who come to Iago for advice and who are manipulated by him: Roderigo in Act One; Cassio in Act Two; Othello in Act Three; Desdemona in Act Four. In each act, a character comes to Iago for counsel. Roderigo wants to win Desdemona for himself and seeks Iago's help. Cassio wants to regain his position as lieutenant and seeks advice from Iago. Othello is sucked into believing that Desdemona is cheating on him and turns to Iago for help in executing Cassio, while Desdemona pleads with Iago to help her win her lord again. With each act, the stakes grow higher as Iago weaves his web of deception of manipulation.
It is in Act 5 that Iago's plans come to fruition, and that the one character who did not come to him for advice and whom he did not try to manipulate exposes him for who he is. This character is his wife Emilia.