Iago utilizes each of these persuasive devices in Othello. Ethos requires the speaker to convince his audience that he is worth listening to and is trustworthy. Pathos depends upon the speaker's ability to appeal to the reader's emotions. Logos demands the speaker to persuade the reader with logic. First, Iago bribes and manipulates Roderigo into betraying Desdemona and Othello--they have been married without her father's permission or knowledge. Next, Iago uses the Moor's race against him, insinuating that he is not worthy and should not be married to Desdemona. Meanwhile, he is consistently manipulating Othello, appearing to be his friend and confidant as he betrays him by playing puppeteer with those around them.
The link listed might be useful in understanding what ethos, pathos, and logos are, but each of these examples exist in the text and fuel the progression of the plot of Othello. Iago demonstrates how dangerous words can be, particularly when woven so cleverly and tied to relationships.
When Iago discusses his dissatisfaction with not being promoted to lieutenant with Roderigo, he uses logic (logos) to render a solid argument. Cassio is a foreigner, inexperienced and a mathematician to boot - surely not the qualities required of a lieutenant, yet he had received the promotion. He effectively also uses ethos: that he had been loyal to Othello, that he had been in battle at his side and that even senior senators had vouched for him. These were all factors which should have been enough to secure him the promotion.
Iago furthermore appeals to Othello's emotions (pathos), by vividly describing how Cassio had declared his love and desire for Desdemona when he had shared a bed with Iago. That Cassio had embraced him and had passionately kissed him in a delirious dream, believing that Iago was Desdemona! This is obviously an odious lie, but Othello is overcome with emotion on hearing the lurid detail.
When Iago advises Cassio to approach Desdemona in an attempt to regain his position after his dismissal, telling him that Desdemona is compassionate and that she has control over Othello, he appeals to Cassio's sense of logic (logos). Since Desdemona is the love of his life, Othello would listen to her. This piece of advice is appealing to Cassio, for it seems to make sense. Little does he realize that this is part of Iago's grand scheme to manipulate events and further demonize both Cassio and Desdemona.
In another instance, Iago cleverly uses rhetorical questions by acting as if he does not know what Othello is talking about. After witnessing Cassio taking leave of Desdemona, 'guilty-like', Iago remarks, 'I like not that.' When Othello later enquires about his comment, Iago feigns ignorance, answering each of Othello's questions with another question, further infuriating Othello and creating even more suspicion in Othello's already confused mind.
It is interesting to note that Iago is intimately trusted by all those with whom he associates. It is only when the damage of his evil manipulation has been done, that the real truth of his Machiavellian machinations are discovered.