Different manufacturer with the same video card?

This has confused me for a while. ATI has different manufacturers like: VisionTek, Diamond, Best Data?, Sapphire. What's with that? Also, which one(s) make good-quality cards? (Which one should I buy from?...) I'm planning to buy a newer card so...yeah.|||ask around and find which company has the best return and warranty policy. thats about the only difference.