Phrases with as many different letters as possible
I remember there was a term for such sentences. Does anyone know what they are called?
"Pangram" is pretty close and may be the word you were thinking of:
A pangram (Greek: pan gramma, "every letter"), or holoalphabetic sentence, is a sentence using every letter of the alphabet at least once. (Wikipedia)
If you are curious about short pangrams, we have some answers for that, too!