Ninja is simply a general name, applied to Kawasaki motorcycles ranging in sizes from 250s to 1400s to signify they are sportier than other models.
ZR-7 is a specific model sold from 1999 to 2005 with a old fashion, antiquated, (KZ750), simple, 4-cyl, air cooled engine.
....if you mean the Versys Ninja 650, then the ZR-7 was a (rushed) stop gap model to compete against the Suzuki SV650, until Kawasaki could develop the Versys to fill that gap..............and the differences are about 30 years of technological advances..
I just read up on it a little. Kawasaki trademarked some form of the word "Ninja" in '84 with the GPZ900 and have re-registered the trademark several times since then. So it's been around for quite a while.