From what I gather, it's when a person is angry about a situation, can't or won't say anything about it so they'll do things that are indicative of someone who is angry. For example:
Stalking around with an attitude because someone didn't empty the clothes dryer. When asked what's wrong, this person will answer with a flippant, 'Nothing', and suddenly "forget" to do something the other person asked.
"Saving up" infractions. Someone who watches what another person does, incorrectly according to their standards might I add, and catch an attitude about it without saying anything, or blowing up and letting that person have it all at once.
Forgetting to do things on purpose. Huffing and puffing, heavy sighing, slamming doors alllll in the hopes the person you're mad at will the picture so they can ask what's wrong, and you can either tell, or do the nothing thing, and go 'round and 'round on the passive-aggressive merry go round.
In other words, doing little things to hint to the person that you're ticked off about something by doing spiteful things instead of just saying,"I'm upset and this is the reason why," like an emotionally health person would do. I'll admit I used to do this, until I saw that it wasn't right so I stopped. When I'm mad I say something. I also don't fall into the trap, if you're mad about something I've done, that's you're business. Until you make it known, verbally, that I have done something to upset you I won't be asking, 'What's your problem?'
I have a friend is unbelievably passive-aggressive, my mother is too.