What is anger? I believe the best definition anybody ever made about anger was Osho: “Anger is active sadness; sadness is inactive anger. They are not two things.”.
We tend to have more respect for angry people, than for sad ones. Anger looks fearless, sadness appears to be weak. Furious people demand fear from others, meanwhile sad people trigger only pity, in best case empathy. But it's only an illusion. Angry people trick others, who have low self-esteem or poor judgment.
Let's just go back and ask again: what does anger mean and reflect? When are we angry? When everything goes just the way you wanted? No. Anger pops up when nothing goes the way you planned, nothing and nobody care about you or your wishes.
Anger, same as sadness, appears when you are not happy, so it is the opposite of happiness.
Anger indeed is not different from sadness. It's only a different manifestation of unhappiness.
Usually, people who are visibly angry, are more extroverted, and people, who look just sad, are more introverted. It's more like a question of character, as a question of different conceptions.
So, don't get fooled by it: not when you see it, not when you are experiencing one or the other on yourself.
Think like this: you have an angry outburst and throw some plates at the wall. You feel as cool? Thinking you showed them all, who the “boss” was? Maybe you did, but other people's apprehension won't change the fact, that you are unhappy. Period. No matter how you manifest yourself.
As a summary: no matter if you beat someone up or go crying alone in the corner of your pink room. The reason is the same. If a car is painted in green, you will not think that is grass, will you?