Why are champs considered 'top, mid, bot' champs and why not?

I'm fairly new so don't hate me on this, but I never understand what makes a champ a "top, mid, adc, sup, jngl' champ. For example if you pick Garen as a sup you get flamed for trollpick. or Xin Zhao adc 'trollpick' I never understood why.
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