Depending which country your in, its a huge difference. "pissed off" is the expression used in America to describe when someone is upset, but it is considered more vulgar and impolite than "upset". It is also not appropriate when someone is really angry or distraught. "He's pissed off someone killed his girlfriend ." is NOT something we'd say. In Britain and Australia "pissed" is used to describe someone who is drunk : "Don't let him drive, he's pissed."

The difference between each word is about how strong it is, for example, "Pissed" is as strong as angry, but "Upset" is less strong than pissed. You can use angry and pissed at the same because those are similar, but if you use upset is less Strong, I mean both of them means almost the same that depends what are you talking about, and the intensity of your feelings. Learning the meaning may help you! Angry - you get mad, usually this emotion comes with emotional disorder including yelling or shouting and perhaps violent behaviors. Upset - Something makes you feel uncomfortable, and you feel emotional pain. Pissed - You feel unfair or excessively annoyed so you're not cooperating with people.

Pissed means to be aanoyed but upset as a verb means to make someone unhappy and as an adjective means unhappy