Design-wise:
The operating element. A needle valve has a tapered shaft (needle) pressed against a seat. Which is lowered or raised by a screw that is connected to the wheel or handle etc the top.
A ball valve has a sphere seated between a gasket or “stuffing”. This sphere/ ‘ball’ normally has a through hole of same bore as that of inlet connection. (Called the full bore design) this ball is connected to a stem that in turn is connected to a handle.
Now, operation-wise:
needle valve is of precision or regulation application.
Ball valve on the other hand is a coarse flow control. Normally just open and close…