Iago's Jealousy

This is going to sound like a homework question, but you all know that it's been a long time since I was in high school :).

Othello is typically described as a story of jealousy. When there's even the slightest hint that his wife has been unfaithful, noble Othello is reduced to a snarling, violent animal who sees her death as the only possible outcome.

But what about Iago's jealousy of Cassio? Doesn't the whole play revolve around something that Cassio has, that Iago wants? It's not just the promotion, is it? Cassio has Othello's attention. When Othello needs something, he turns to Cassio. Iago wants to be in that position. I would say that Cassio has Othello's love, but I'm not sure how accurate that is -- Othello catches Cassio brawling at the bar and demotes him just as quickly. He doesn't seem to lose too much sleep over it.

How are the two different? How are they the same? Discuss.