PCIe x1 slots can be used for USB3-to-PCIe adapter cards, sound cards, ethernet adapter cards, etc.
PCIe-2 x16 slot can be used for a second graphics adapter card for crossfire or SLI connection, or for any PCIe card ranging from PCIex1 to PCIe x16, i.e. you can fit in a PCIe x16 slot a PCIe x1 card, a PCIe x4 card, up to PCIe x16 card. PCIe slots are backward compatible regarding link width, I mean x1, x2, x4, x8, x16.
I think your M/B has two slots for graphics card installation. You can find the info in the M/B manual.
Hope this helps.