Passive aggression is when you convey a negative message to someone in a very indirect manner. It is, as @Espiritus_Corvus and @Dutchess_III said, a cowardly choice, and it earns no one’s respect.
Here is a classic example:
You know a specific person in your workplace ate a yoghurt that you left in the office refrigerator, but instead of going to that person and asking him or her not to eat your yoghurt again, you leave a big note in the kitchen saying, “COULD PEOPLE NOT EAT OTHER PEOPLE’S FOOD,” or whatever. So, instead of targeting the person you know is to blame (which might make you feel uncomfortable), everyone in the office is made to feel uncomfortable, and the person who ate your yoghurt can pretend he or she had nothing to do with it. And you still don’t have any yoghurt.
If you have the choice between doing something passive-aggressive and choosing to have a more direct interaction, choose directness. No one likes people who are passive-aggressive.