What is the difference between 'divert' and 'avert'?

divert – to take an action that causes something to change direction
avert – not as active a word as divert… it’s the act of turning away from, or prevent a calamity

The major difference is that avert is always related to a negative thing [averting something like an accident], whereas divert is more neutral [diverting the canal water into two pathways].

I think there’s an object difference for me which is the most important aspect – you are diverted, and you avert something.

Well, they can both be transitive verbs. That is, you can divert a bullet with a shield or you can avert it by changing the mind of the shooter. I think the big difference is in “force”. Diverting a bullet with a shield, or diverting the course of a river, can take a lot of effort and force. But averting a thing—at least in the way that I see the word used more often—is a process of negotiation and avoidance rather than brute force.

Generally agreeing with @squirbel and @CyanoticWasp, I would also add that to divert something causes it to go elsewhere, whereas to avert it prevents it from happening. When you avert a calamity, it isn’t that the calamity moves aside. The calamity doesn’t occur.

You can divert a physical object (as well as things that are not physical), but you can’t avert a physical object. You avert an event or occurrence.

Hence you can divert traffic to avert a jam. You can divert someone’s attention to avert an argument. You can divert a river to avert a flood. You can divert the cops to avert discovery of a crime in progress.

@jeruba worded it far better than I. :)