Samsung makes their own complete modules, which will have a sticker with the blue Samsung oval logo. You can't buy direct from samsung, you have to purchase through a reseller. This would be called Samsung Genuine or Original. Also, Samsung makes chipsets that other third-party companies assemble onto their own modules. This would be called Samsung-on-Third.
Don't get too hung up on the brand, choose a reputable seller who guarantees compatibility and will stand behind the RAM, of whatever brand.
There will be absolutely no Speed difference between brands of RAM in the Mac.
*(with a caveat about the new Corsair CL4 RAM, about which the jury is still out. We are sending some to Barefeats.com for testing)