Why do they call Holdens, Pontiacs & Vauxhalls?

I dont understand LOL in the pontiacs category theres holden cars with different names. I have never heard of a PONTIAC or a VAUXHALL in my entire life and now theres images of holden commodores, holden barinas named as other cars and makes? how weird is that. well the true name is HOLDEN america!!! lol