The ASRock Fatal1ty 990FX Killer comes with three PCI Express 2.0 x16 slots and two PCI Express 2.0 x1 slots.
Two of the PCI Express x16 slots (PCIE2 and PCIE3) always work at x16 speed, and the other slot (PCIE5) works at x4. Therefore, when installing two video cards, place them in the first (PCIE2) and third (PCIE3) slots for best performance.
If you plan to install a dual-slot video card in the third PCI Express x16 slot (PCIE5), you will have to buy a case with at least eight slots. (Computer cases usually have seven.)
The PCI Express x16 slots support both SLI and CrossFireX technologies.
The Fatal1ty 990FX also comes with an M.2 slot (a.k.a. NGFF), supporting SSDs that use this new form factor. This slot supports SATA-600 and PCI Express x2 interfaces.