Dash: -
Em Dash: -- (there's probably a way to connect it on this platform, but imagine those are connected; basically, it's a longer dash).
Ordinarily a dash is used to separate a number or a letter, or something else, as in: 1 - Ohio State
An em dash is used to set off a clause within a sentence, and most style guides call for no spaces between it and the words around it, e.g.,: I loved the beauty--not the drugs and homeless people--in San Francisco.
The idea of a clause set off with em dashes is that it has more emphasis than a parenthetical or a clause separated with commas.