Othello's right-hand man.  In Act 2, Scene 3, he uses Roderigo to get Cassio drunk and to pick a fight with him, and then when Iago has to relay what "really happened" to Othello, he tells him that Cassio attacked Montano seemingly for no other reason than because he was rash and drunk.  Othello fires Cassio on the spot.

2.  In Act 3, Scene 2, when Iago and Othello first enter the garden, Iago draws Othello's attention to Desdemona talking to Cassio, and says, "Ha! I like not that."  With those simple words he gets Othello to start questioning why it is a problem for his wife to talk to Cassio.

3.  In the same scene, Iago pushes the issue farther by reminding Othello that Cassio and Desdemona would have had plenty of time to spend together because back in Venice, Othello used Cassio as his messenger when he was courting Desdemona.

4.  Because Iago has already orchestrated a situation where Desdemona will talk to her husband about Cassio, he tells Othello, "Look to your wife; observe her well with Cassio."  He knows that Othello will not be swayed with mere words; so he sets up a situation where Othello will "see" Desdemona's unfaithfulness.

5.  The most important seed of doubt is, of course, the handkerchief incident at the end of Act 3.  By planting the handkerchief with Cassio and causing Othello to notice that Desdemona does not have the handkerchief, Iago not only causes unwarranted suspicion in Othello's mind but also provides the "ocular proof" that Othello needs.

Iago's steps to arouse Othello's suspicions are psychologically and emotionally manipulative, which might explain why they are so difficult to identify. Iago is sneaky, and his sneakiness is what makes his villainy so uniquely evil.

Iago manipulates Othello's sense of himself as a man in order to make Othello feel suspicious of Desdemona. Iago also uses masculinity in other ways to involve other characters in his scheming. First, Iago sets up a situation where Cassio drinks too much and misguidedly feels a need to prove his manliness in a brawl; this situation causes Cassio to fall out of Othello's favor, and when Othello fires Cassio in disgust for this showy bad behavior, Othello is scornful and punishes Cassio for his supposed need to prove himself as a man.

This situation makes Cassio's suspected involvement with Desdemona even worse for Othello, as Cassio is made out to seem like a man supremely unworthy of Desdemona. And worse, how could Desdemona, who has sworn her love for a hero, for a real man like Othello, cheat on him with a weaker man like Cassio? This insult to Othello's sense of masculinity makes his suspicions even more painful, all thanks to Iago's cunning manipulation of the two men's emotions.

Iago primarily uses cuckoldry (or the fear of it) to undo Othello. At the time the play was written, men were terrified that their wives would cheat on them; it was seen as emasculating. Othello, as a black man, already feels inadequate in a predominantly white society, so it is not difficult for Iago to plant the seed that perhaps Othello is inadequate as a man, as well. He achieves this by targeting Othello's lieutenant, Michael Cassio. Iago instigates a drunken brawl that was considered unseemly for a man of Cassio's standing, so Cassio was stripped of his rank. Iago then goes to Cassio and suggests that approaching Othello about regaining his rank would be unwise, but talking to Othello's wife, Desdemona, would be more productive.

Iago times it so that Othello sees Cassio and Desdemona talking, instantly raising his suspicions. He then hints to Othello that Cassio has a womanizing reputation, only furthering Othello's suspicions that Cassio and Desdemona are having an affair. But as these are only suspicions, Iago has to take it a step further. He convinces his wife, Emilia, who is Desdemona's lady-in-waiting and desperate to please her husband, to steal the handkerchief Othello gave Desdemona and make sure it ends up with Cassio.

When Othello sees the handkerchief in Cassio's possession, it is the final straw, and he decides that he is being cuckolded.

I'll try to explain Iago's complicated plot in easy-to-understand steps.

Step 1: Get Cassio, Othello's right-hand man fired, so that Othello no longer views him as honorable. (Iago does this by getting Cassio drunk and involved in a brawl.)

Step 2: Convince Cassio that the only way he can get Othello's trust and respect back is to plead his case before Othello's wife Desdemona so that she will talk to Othello.

Step 3: Arrange to be walking with Othello when they just "happen" to see Desdemona and Cassio talking quietly.

Step 4: Make suggestive comments to Othello about Cassio's way with the women and his relationship with Desdemona.

Step 5: Warn Othello to watch his wife closely (so that he will notice all the ways in which Iago plans to frame Desdemona and Cassio).

Step 6: Provide the "ocular proof" that Othello asks for.  At the end of their conversation, Othello is not entirely convinced that Desdemona would be unfaithful to him; for all that Iago has provided so far are innuendos and lewd observations.  The visual proof ends of being the handkerchief that Othello lovingly gave to Desdemona ending up in Cassio's hands.

I hope that this helps.  Iago is the perfect two-faced character, and his plot to bring down Othello and Cassio involves master manipulation and numerous characters; so I know that it gets confusing in Acts 2 and 3.