Motherboard expansions slots: PCI Express X16 lanes (above) and 2 PCI Express X1 lanes (below)

Dual PCI-Express slots for SLI graphics cards from NVIDIA; a small PCI Express x1 Lane slot has somehow blundered into the space between them.

As a serial bus, PCI Express should not be confused with PCI-X or plain vanilla PCI. These latter types use parallel signaling.

PCI Express (PCIe) is the latest interface for graphics cards; though in principle it is also usable for other peripheral components, at the present time there aren't any other mass market components that use this bus. On paper, PCIe X16 offers almost twice as much bandwidth per stream as does AGP 8X. But in practice, this advantage is not yet fully exploited in graphics cards available today.

AGP graphics card (above) compared to a PCI-Express graphics card (below)

From top to bottom: PCI Express x16 lanes (serial), two parallel PCI and PCI Express x1 lanes (serial)