Can you only cast a spell once per day or can you cast them over and over?

@ChrisPerkinsDnD confused on how spells work. Can you only cast a spell (like harm) once per day or can you cast them over and over?

— Nick Wood (@HiMyNameIs_Nick) July 9, 2016

You can cast a spell multiple times if you have the spell slots. Each casting expends a slot of the spell's level. https://t.co/0qz1cU9BI8

— (((Chris Perkins))) (@ChrisPerkinsDnD) July 9, 2016