When do you know when to use parenthesis versus a dash? Or is the dash called a hyphen? What's the difference?

Parentheses insert a relevant clause into a sentence, but if the parenthetical clause is removed, the sentence still makes perfect sense. The em-dash (represented by two hyphens or by one long dash) can (if you use only one) transition to a supporting clause at the end of a sentence or (if you use two) insert a supporting clause into the middle of one. A single hyphen doesn't interrupt a sentence in this manner so much as join two words or names together. "Charlotte-Sophia ruled the city-state of Murburg," for instance.

The main difference between parentheses and em-dashes is, you must use an even number of parentheses in a sentence, but you have the option of using only one em-dash in the middle of a sentence as a transition between the main idea and the supporting clause: "I despise tonic water--I only drink it for its health benefits." You can also use two of them, provided you plan to get back to the main idea you transitioned away from: "I can't drink coffee--it gives me kidney stones--but I love the way it smells!"