What is a Pci ex X16 slot on my mother board?

I have your standard Pci 1, Pci 2 and Pci 3 slots for add ons but where my AGP slot is suppose to be is a Pci ex X16 slot. None of my graphic cards fit into this slot. I think I read somewhere that it is used for upgrades. If it is , what kind of upgrades and what kind of card fits into this slot. The reason I ask is because my graphic program is intergrated into my mother board along with audio but my graphics aren't to cool on playing back movies downloaded from the enternet so I was going to ad a better graphics program, but I don't have a AGP slot for it.