Describe the relationship between Othello, Desdemona, and Cassio in Othello.

Othello's relationship with Desdemona is notable for its passionate intensity. Initially, that's a good thing, but as the play progresses it helps to contribute to their downfall.
The marriage between the Moor of Venice and the white noblewoman is something of a mismatch in this class and race-conscious era. Yet Othello and Desdemona are so headstrong that they simply don't care what society thinks. Nevertheless, there's a deep well of insecurity lurking beneath Othello's assured surface. And it begins to be revealed when Cassio arrives on the scene. Cassio undoubtedly has a thing for Desdemona, but he respects Othello too much to act on his desires. Besides, Desdemona is unfailingly loyal to her husband, and though she'll intercede with Othello on Cassio's behalf, that's as far as she's prepared to go. She and Cassio are just good friends and nothing more.
Desdemona's act of kindness is enough, however, for Iago to seize his opportunity. He turns a perfectly normal gesture of friendship into evidence of an illicit affair. He poisons the well of Othello's insecurity until the Moor is convinced that his wife has been cheating on him with Cassio. She hasn't, of course, but Iago is such a master manipulator that Othello comes to believe every lie he tells him. And so the passionate intensity that was always such a notable hallmark of Othello and Desdemona's love is expressed in a final, terrible act of murder as Othello kills Desdemona in a jealous rage over her non-existent adultery.