It should be added: In most cases for spells that has a level:
They generally do full damage on a failed save, and half damage on a passed one. And if the save is failed they usually add some sort of bonus (Like Thunderwave)
This doesn't apply to cantrips like Sacred Flame, which is straight up a "Save or Damage" spell.
Vennumz puts it very well. Just want to add that it's always your class spell ability that is used to determine the DC of your spell. Wizards use Int, Clerics use Wis and Warlocks use Cha.
Further more, there's a difference between a save and an ability check. So spells like Hex that can affect ability checks, doesn't work on Saves.
