What is NDJSON?
NDJSON is one JSON object per line, and nothing wrapping them. That single change makes it streamable - a program can handle a line at a time instead of holding the whole array in memory - which is why logs, data exports and machine-learning datasets ship in it.
What is JSON?
JSON is the shape data arrives in when it comes from an API. Tabular data usually lands as an array of objects, one per row, with the column names repeated as keys in every single one - verbose to read, but unambiguous.