The ASUS Z97-PRO comes with two PCI Express 3.0 x16 slots, one PCI Express 2.0 x16 slot (working at x4 or x1 speed), and four PCI Express 2.0 x1 slots.
The first two PCI Express x16 slots are controlled by the CPU, so it is based on the 3.0 specification. They share bandwidth, so if the second one (PCIEX16_2) is empty, the first one (PCIEX16_1) works at x16 speed, but if it is populated, both work at x8 speed. On the other hand, the third PCI Express x16 slot (PCIEX16_3) is controlled by the chipset (so it is based on the 2.0 specification) and shares bandwidth with the extra USB 3.0 ports and with the fourth PCI Express 2.0 x1 slot: if one of them is used, the third PCI Express x16 slot works only at x1 speed, not x4.
The motherboard supports CrossFireX and SLI arrays.
There is also an M.2 slot, supporting one SATA-600 or PCI Express SSD module.