Does porsche own other car companies?

Hi friends, this is my second thread. Actually i had a debate with my friends on Porsche and other car companies. As far what i heard was that Porsche is a parent company of Audi and i was insisted. But my friend wasnt accepting, he kept saying these are two different companies. I dont know who is right. Please can someone put focus on this? Also I am thankful if someone gives me a list of such parent companies owning other companies.