The theme of jealousy is central to Shakespeare's Othello. Jealousy drives the plot and subplots forward, and it is the primary motivation for the central conflict of the play.
IAGO: O, beware, my lord, of jealousy!
It is the green-eyed monster, which doth mock
The meat it feeds on. That cuckold lives in bliss
Who, certain of his fate, loves not his wronger;
But O, what damned minutes tells he o'er
Who dotes, yet doubts, suspects, yet strongly loves!
OTHELLO: O misery! (Act 3, scene 3)
By the time Iago warns Othello against jealousy, the "green-eyed monster" is already feeding on Othello's mind and inflaming his emotions.
Everything that Iago tells Othello and everything that Othello sees with his own eyes—as orchestrated and manipulated by Iago—leads Othello to believe that Desdemona is unfaithful to him with Cassio. Othello is blinded and enraged by his jealousy, and he ultimately kills Desdemona as a result of this all-consuming jealousy.
Othello isn't the only character in the play affected by jealousy. Roderigo is a relatively minor character, but his love for Desdemona and his jealousy of Othello contribute to the subplot in which Cassio loses his position as Othello's lieutenant. Roderigo goads Cassio into a fight, which leads to his demotion. This causes Cassio to ask Desdemona to speak to Othello on Cassio's behalf, which only strengthens Othello's suspicions about Cassio and Desdemona, further inflaming Othello's jealousy.
Roderigo's jealousy also extends to Cassio, whom Roderigo tries to kill because of his apparent closeness to Desdemona.
Iago, too, suffers from jealousy that eventually leads to his demise. Iago reveals his jealousy of Cassio in act 1, scene 1, when Iago learns that Othello has chosen Cassio rather than Iago to be his lieutenant.
IAGO: I know my price, I am worth no worse a place.
... This countercaster [Cassio],
He, in good time, must his lieutenant be,
And I—God bless the mark!—his Moorship's ancient.
Iago's jealousy and envy of Cassio contributes directly to the Cassio-Desdemona subplot, which leads to Othello's jealousy and contributes to Roderigo's jealousy of Cassio.
Iago also harbors jealousy against Othello for what Iago suspects is his wife Emilia's infidelity with Othello.
IAGO: And it is thought abroad that 'twixt my sheets
He has done my office. I know not if't be true;
But I for mere suspicion in that kind
Will do as if for surety. (Act 1, scene 3)
This suspicion of his wife's infidelity clearly drives Iago's plot against Othello, but it also reveals Iago to be jealous of Othello's marriage to Desdemona, toward whom Iago seems to have feelings beyond pure physical attraction.
IAGO: Now, I do love her [Desdemona] too,
Not out of absolute lust, though peradventure
I stand accountant for as great a sin,
But partly led to diet my revenge,
For that I do suspect the lusty Moor
Hath leap'd into my seat; the thought whereof
Doth like a poisonous mineral gnaw my inwards,
And nothing can or shall content my soul
Till I am even'd with him, wife for wife;
Or failing so, yet that I put the Moor
At least into a jealousy so strong
That judgement cannot cure. (Act 2, scene 2)
Iago incites Othello's jealousy to such an extent that Othello's "green-eyed monster" consumes him and leads him to kill Desdemona.
Iago, too, is consumed by his own "green-eyed monster," which causes Iago to commit hateful acts that lead to the deaths of Desdemona, Othello, Emilia, and Roderigo—and which will likely lead to Iago's own torturous death, to be supervised by Cassio, the first target of Iago's jealousy and sole survivor of Iago's villainous plots.
LODOVICO: [To Cassio] To you, Lord Governor,
Remains the censure of this hellish villain,
The time, the place, the torture. O, enforce it! (Act 5, scene 2)